Some verbal inaccuracies which had crept into the first translation have been corrected, and in every respect the editor may be congratulated on the work in its present form. It will be of the greatest use to students-especially, perhaps, to those who have to work alone.

D. H. S.

Traité d'Optique. Par M. E. Mascart. Tome I. (Paris: Gauthier-Villars, 1889.)

THIS is the first half of a very elaborate treatise on optics, the full scope of which we cannot tell till the second volume appears, as no hint is given of what is yet to come. This first volume begins with the fundamental principles of the wave-theory of light, deduces from them the elementary laws of geometrical optics, discusses the properties of a co-axal system of refracting surfaces, describes the structure of the eye, expounds the facts of colour-mixture, points out the conditions which determine the resolving power of a telescope, develops at great length the theories of diffraction and interference, with some of their principal applications, and devotes about 80 pages to polarization and double refraction. There is practically nothing about the microscope, and nothing at all about the paths of rays in media of continuously varying density.

The book is by no means easy reading, and the labour of perusing it is increased by the smallness of the reference letters (with their numerous accents and suffixes)

which occur in the figures. The plan involves much specialization. For instance, the proof of the formula for retardation on which the theory of Newton's rings depends is not given in the sections devoted to Newton's rings and colours of thin plates, but some 370 pages earlier. In many cases, when the student has found a formula which appears to contain the information of

which he is in quest, he has to search carefully through a long series of preceding pages before he can find the meaning of some symbol which occurs in it. The volume contains a vast store of information, but not generally in a form to suit hasty seekers after truth. It requires to be studied at leisure, and the time so spent will not be wasted. Great pains have obviously been taken to embody the latest information and present it in the clearest form. We may instance the spiral curves which illustrate the values of Fresnel's integrals, and the curve (to which a folding-plate is devoted) showing the relations of the colours of diffraction fringes to the three primary colours. There is an excellent discussion of the theory of concave gratings, both for reflection and refraction. The least attractive chapter is that entitled "Properties of Vibrations." It is a discussion of the composition of simple harmonic motions, and occupies 40 pages bristling

with elaborate formulæ. We think a more moderate display of mathematics under this head would have sufficed.

The order of arrangement adopted in the volume is rather peculiar, and baffles all a priori conjecture. For instance, the discussion on colour-mixtures occurs in a chapter on "Interferences," and the investigation of the conditions which determine the resolving power of a telescope is given in the introductory chapter under the head of "Preliminaries."

The book is essentially a mathematical treatise, all experimental descriptions being reduced to the narrowest possible limits.

The preface states that the work is addressed mainly to "pupils of the Faculties and Schools of higher instruction," but we think its principal use in this country will be as a book of reference for teachers. Its value for this purpose will be greatly increased if a good alphabetical index is added at the end of the second volume.

J. D. EVERETT.

Bibliothèque photographique: Le Cylindrographe, Appareil panoramique. Par P. Moëssard, Commandant du Génie breveté, attaché au Service géographique de l'Armée. (Paris: Gauthier-Villars, 1889.)

THIS is a description of a photographic camera invented by Colonel Moëssard, in which the lens is pivoted on an axis, and the sensitive film is arranged in a cylindrical form about this axis, on a radius equal to the focal length of the lens. By this means a panoramic view of angular breadth up to 170° can be taken. The camera being fixed in position, the lens is uncapped, and then rotated quickly or slowly, according to the speed of the plate, and the intensity of light in any direction. The author claims for the instrument useful employment in surveying, either in the carefully detailed plans of an ordnance survey, or in the rapid views useful for warlike purposes, which the instrument can afford. Two photographs taken with the aid of the instrument illustrate very favourably its powers, especially for architectural purposes.

A Hand-book of Modern Explosives. By M. Eissler. (London: Crosby Lockwood and Son, 1889.)

IN this book the author of "Modern High Explosives" has collected much useful information about the various explosives now in use. The greater part of the work is devoted to nitro-compounds, but short accounts of the other types of explosives now being manufactured are added. The manufactures of gun-cotton and nitroglycerine receive full treatment, together with the modifications introduced in the various large factories both of America and Europe. The important subject of the use of explosives in fiery mines has a chapter to itself. The description of the tests of flameless powders is of especial interest; in fact, the official reports of the tests of many of the most important explosives are perhaps the most instructive portions of the book. The chapter

dealing with the practical application of explosives should be useful not only to the miner, but also to officers of both services to whom blasting and the use of explosives generally may at any time become a necessary auxiliary. An interesting account of the history and trials of the Lalinsky gun, together with the manufacture and use of gun-cotton shells, is also well worthy of their perusal. Little is said on the use of explosives below water, especially on the subject of the removal of wrecks, which Four appendices are would stand far fuller treatment. added, two dealing with the analysis and determination of stability of explosives, and one containing abstracts from the principal provisions of the Explosive Act of 1875. Although there is much that is necessarily old, still this is a book that will be read with interest by most who are tions are well executed, and the whole wonderfully free accustomed to work with high explosives. The illustra

from printer's errors.

LETTERS TO THE EDITOR.

[The Editor does not hold himself responsible for opinions ex pressed by his correspondents. Neither can he undertake to return, or to correspond with the writers of, rejected manuscripts intended for this or any other part of NATURE, No notice is taken of anonymous communications.]

The Peltier Effect, and Contact E. M.F. WITHOUT any further reference to the heading of a letter on p. 102, signed The Reviewer," I wish to discuss an interesting argument therein propounded as proving that a true electromotive force at contact between two metals cannot be the cause or sole cause of the Peltier effect, unless the latter be simply

proportional to absolute temperature. The argument is very like one that I indistinctly remember to have heard suggested some time ago by Prof. Schuster, and it struck me at the time as ingenious and not easily answerable.

(3) That vacuum has a specific inductive capacity.

Grant all these, and the argument is sound. Decline to admit any of them, and it proves nothing. Break down the first two of them, and it proves too much: it proves the nonexistence of any thermal contact-force whatever between conductors. For if there were any E.M. F. at the metallic contact, and none at the other or vacuum contacts, a continuous current would flow, propelled by energy derived from a cold place.

This argument is indeed the ordinary one to prove that the algebraic sum of the E.M.F.'s at all the junctions of a closed conducting circuit in which no energy but heat is supplied must be zero when the temperature is uniform.

The proof scarcely holds when insulators are interposed, though the fact may be true nevertheless. When chemically active substances with their extraneous supply of energy are interposed, the fact itself is no longer true. But how do we know what is true when vacuum is interposed? The hypothesis on which the argument is founded is a baseless conjecture.

But it may be said, Are not the hypotheses probable? Do you not yourself believe them? I believe in (1) and (3) provisionally, but certainly not in (2). The contact E. M. F. between two substances is probably some surface action or skin phenomenon, and I see no reason why it should not occur as well in the boundary between metal and void as in the boundary between one metal and another. Indeed, it is not improbable that the sum of the E.M. F.'s in every circuit of chemically inert substances, whether conducting or not, and inclusive of vacuum, is zero under uniform temperature conditions.

All that is wanted to establish this is the knowledge that in a circuit of any one substance at non-uniform temperature the total E.M. F. shall be zero,' or that the Thomson effects in a single substance always balance each other; i.e. that the total E.M.F. in a circuit shall depend on a potential function of temperature, or dE= ƒ'(t)dt.

Now it is quite true that this f'(t) is the Peltier coefficient divided by absolute temperature, and that f() in its most general form contains an arbitrary constant, but what of that? Nothing is known of ft) except that it is a potential function: it is not known to represent any physical effect. I never said that the Peltier effect enabled us to find the most general form of the function (); I said it gave us the E. M. F. at a junction.

And there is much ground for the assertion; for it is easy to show that in a simple AB circuit, with junctions at t1 and ty the total E. M. F. is

just as if the resultant E. M.F. were the algebraic sum of two Peltier E. M. F.'s and of two Thomson E. M. F.'s.

My only contention is that this equation, which is undeniably true when the П are interpreted as heat-coefficients, is also true and immediately interpretable when they stand for contact E.M.F.'s. The burden of proof as to the physical existence of an unnecessary and in every sense arbitrary constant rests with those who doubt this simple explanation.

It is difficult to see how a doubt can arise, or how the Peltier and Thomson productions or destructions of heat can be accounted for without local E. M.F.'s. Nohow, so Dr. Hopkinson has proved, and I also have insisted (Phil. Mag., October 1885, 2nd March 1886), except by some wildly gratuitous assumption of an actual physical specific heat for electricity, dependent on the temperature and on the metal in which it happens to be. Liverpool, December 14, 1889. OLIVER J. Lodge.

Mirages.

THE article in NATURE of November 21, 1889 (p. 69), recalls to me mirages I saw in March 1888, while travelling in the East on the steam yacht Ceylon.

On the 29th we were crossing the Black Sea from Sebastopol. 1 Hopkinson virtually pointed this cut, Phil. Mag., October 1885.

It was a fine cool day and quite calm. In the afternoon a false or mirage horizon about 3° above the true one was visible for a few hours. No objects were within range of vision. The mirage disappeared as the sun declined.

The next day was very much warmer, and we saw a more marked and interesting mirage in the afternoon as we were steaming across the Sea of Marmora away from Constantinople. In this case it appeared only in the west, and objects were seen reflected in an inverted position. A small conical-shaped island was seen with its inverted image at times distinct from and at times blending with the original. The image was distinctly seen of some land, which was actually below the horizon. The mirage of the reflection of the sun in the sea was, when seen through a glass, especially beautiful. It resembled a glorious cataract of golden water. This mirage lasted till quite the dusk of the evening, and then gradually thinned down and died

I AM very sorry that I took no notes, some six or seven years ago, on the first and only occasion of my seeing self-luminous clouds, but though I can give neither date nor positions, the following facts are still fresh in my memory.

Passing through Bushey Park after dark, I noticed an aurora borealis, and, as I had only recently seen the rather rare phenomena of the rays of the setting sun converging towards a point in the east, I followed the direction of one of the principal beams of light towards the south, when, at a point somewhat south of my zenith, I noticed an equatorial belt of luminous clouds. I found that each cloud belonged to a ray, and faded and brightened with it, but was separated by about 60° of clear sky. This belt of clouds extended down to the western horizon, the eastern one was obstructed by trees, while shortly afterwards small dark clouds appeared on that side, and the sky soon became overcast.

The luminous clouds were quite transparent, so that even faint stars could be seen through them when at their brightest. I have heard from Scandinavian captains that these luminous belts are sometimes seen in northern latitudes, and are sure signs of bad weather. I have written these few remarks in the hope that those of your readers who may have the chance of seeing an aurora borealis will also look out for these clouds, and if possible determine their position. C. E. STROMEYER. Strawberry Hill, January 4.

The Revised Terminology in Cryptogamic Botany. THE anglicized forms of most of the terms in common use, employed in the "Hand-book of Cryptogamic Botany" recently issued by Mr. G. Murray and myself, have not up to the present time found much support from our fellow-botanists. I propose, therefore, to give, in some detail, the reasons which have

induced us to adopt them, and to urge their general use on writers on cryptogamic botany. For this purpose we will take as our text extracts from three reviews of the "Hand-book," marked, as all the critiques have been, with only one or two exceptions, by a generous appreciation of the difficulties of our task, and a too great leniency to the many shortcomings of the work :-" The most conspicuous, though not the most important, of these [changes] is the adoption of anglicized terminations for Latin

and Greek technical words. This is a matter in which it is hard to draw the line aright. As a matter of taste we think the authors have gone much too far in this direction. They complain of the awkwardness and uncouth form of these words'; we should have thought the reproach applied much more strongly to 'coenobe,' 'sclerote,' 'nemathece,' and 'columel' (NATURE). "An Englishman may guess what 'archegone' is short for, for example; but why puzzle a foreigner with a new form of a word with which he is familiar in every treatise hitherto written on the special subject in any European language?" (Academy). "Too sanguine expectations complete failure of the somewhat similar experiment made by on this head might well be toned down by remembering the Lindley. . . . Primworts, spurgeworts, bean-capers, and hip

purids are decidedly simpler, even if less euphonious, than Primulaceæ, Euphorbiaceæ, Zygophyllaceæ, and Halorageæ; yet the longer Latin terms are still universally used, while the quasiEnglish ones have never obtained even temporary acceptance" Journal of Botany).

The last of these criticisms appears to rest on a confusion between the principles of nomenclature and those of terminology. In nomenclature, rigid rules have been laid down, and accepted by all leading naturalists of all countries, in order that the scientific names of species, genera, orders, &c., may correspond in scientific treatises in all languages. In the terminology of flowering plants no such rule has ever been attempted to be laid down; but each writer, when writing in his own language, uses terms, usually of classical origin, and derived from common roots, but of a form as far as possible amenable to the laws of the language in which he writes. All that we are contending for is the extension of the same principle to cryptogamic botany; one of the main objects in the publication of our "Hand-book "being to make the study of flowerless plants as attractive to the public at large as is that of flowering plants.

In order to show how recent is the universal adoption of this practice in phanerogamic botany-a change largely due to the influence of Dr. Lindley's writings-we append a list of a few terms in use in standard works of original research or of reference, published within the last thirty-two years, which presented themselves the first to our hand; viz.-" The Miscellaneous Botanical Works of Robert Brown" (1866); Mr. Currey's translation of "Hofmeister on the Higher Cryptogamia, &c.) (1872); Berkeley's "Introduction to Cryptogamic Botany (1857); and Bentley's "Manual of Botany" (2nd ed., 1870) :

Stipula

Brown

Stamina (plural) Brown

Currey

:

With the exception of words which have been incorporated into our language, such as corolla, nucleus, &c., comparatively few of those used in describing flowering plants now retain their classical forms; the most conspicuous exceptions being those applied to the structure of tissues, such as epidermis and those ending in enchyma; and can anything be more puzzling than the forms in common use for the terms derived from the Greek dépua-epidermis, hypoderma, and periderm? We have no doubt that, had our critic lived in the days of Robert Brown and Lindley, he would have thought all the innovations introduced by the latter "uncouth" simply because we were not used to them; and would have said that Lindley had “ gone much too far." In some of those adopted by ourselves we have, in fact, been forestalled by others, as in the cases of antherid and archegone by Lindley, and sporange by Oliver.

We now come to the charge made by our critic in the Academy, that the terms we have introduced would "puzzle foreigners." Unfortunately, our polyglottism, or rather oligoglottism, will not allow us to vie with our reviewer in his acquaintance with every European language; we are compelled to confine ourselves chiefly to three; but these include by far the greater part of European botanical literature -in fact, every treatise which nine out of ten English readers will wish to consult in the original. The statement quoted above seems to have been rashly made.

In Italian, as far as our knowledge goes, the practice is absolutely uniform: no botanical writer of repute uses the classical forms; but every technical term has its Italian spelling and termination. To such an extent is this adaptation to the laws of orthography of the language carried, that we find "xylem" converted into xilema, "phloem in to floema, "hormogonium" into ormogonio, and " hypha" into ife; and this by the first writers "on special subjects.'

Our acquaintance with Swedish, Danish, Dutch, and Spanish is too slight to allow us to speak with confidence; but in all these the general practice is, we believe, the same as in Italian, though not to the same extent; with the best writers, when writing in their own language, the use of terms with Latin or Greek terminations appears to be the exception rather than the rule.

In French, the practice is by no means so uniform as in

We do not mean that these words are exclusively used by the writers quoted; it is not uncommon to find the Latin and the German form used indifferently on the same page. It is noteworthy also that even the most rigid conservatives do not use the Latin form in the plural of such words as "oogonium,' sporangium,' 'antheridium," "sclerotium," &c., but always the German forms, Oogonien, Sporangien, Antheridien, Sklerotien, &c.; such words as "oogonia, 'sporangia," "antheridia,' "sclerotia," &c., are, as far as our experience goes, to be found only in English and American writings and in Latin diagnoses.

9766

Analyzing, therefore, the statement that the Latin and Greek forms of words used in cryptogamic terminology are “familiar in every treatise hitherto written on the special subject in any European language," we find that in Italian the practice is unanimously, and in French (as also, we believe, in most other European languages) preponderatingly in the opposite direction; and that German is the only widely read language of Continental Europe in which even the weight of authority is still on that side.

There are some terms in which, no doubt, the classical form must be retained, especially those which, when deprived of their

66

classical termination, become monosyllabic, such as "thallus," "sorus," "hypha," and "ascus," just as we still speak of a "corolla," a "stigma," a "hilum," and a raphe." But, with regard to the great majority of terms in current use in descriptive cryptogamic botany, we entertain not the smallest doubt that the change will gradually be brought about which has, within the last forty years, become established in phanerogamic botany; and we would venture to suggest to our fellow-workers in cryptogamic botany in this country and in America, whether it will not be best to accept it frankly once for all. ALFRED W. BENNETT.

Exact Thermometry.

I AM quite in agreement with Prof. Sydney Young (NATURE, December 19, p. 152), that after the lapse of a sufficient time— let us say, an infinite time-the constant slow rise of the zeropoint of a thermometer at the ordinary temperature will attain a definite limit; but I cannot accept his view that the effect of heating the thermometer to a high temperature is simply to increase the rate at which this final state is approached. If the results of experiment at the ordinary temperature be expressed in a mathematical formula which admits of making the time infinite, the limiting value of the rise (on that condition) will not exceed on the average 2° C., even in a thermometer of lead glass. After exposure to a high temperature, and in the same thermometer, so great an ascent as 18° C. is a possible measurement, actually realized. The two phenomena are therefore very different in their nature.

The view that, owing to the more rapid cooling of the outer parts of the bulb after it has been blown, the inner parts are in a state of tension, and that it is the gradual equalization of the tension throughout the glass that causes the contraction, has frequently been held, and will probably be for a long time the favourite hypothesis upon the subject. It breaks down, however, when we attempt to calculate what the amount of the contraction might be, on the supposition that it is well founded: only a very small portion of the contraction could be thus accounted for. I regret that I cannot now conveniently refer to Guillaume's interesting demonstration of this result.

Prof. Young has placed on record an experiment with three thermometers, which he heated to 280° C. The zero movement,

however, only ranged from 1° to 12,-small readings which

might very possibly have been obtained, or not, on either of the thermometers at other times. It is consequently very difficult to draw any inference from this experiment. I may, however, mention that closed thermometers made of lead glass are very apt to show a rise of zero after heating to about 120° C. and upwards to some temperature in the neighbourhood of 270° C., and after that a descent of zero; the temperature of 280° C. would in that case be an unsatisfactory one for a test experiment, and the effect of plasticity might very possibly be masked. On the other hand, if the three thermometers were of hard glass, all the zero movements would in that case be greatly diminished, and the results would be in less bold relief.

I do not know any substance more curious or interesting in its properties than glass; and I should be glad if Prof. Younginto whose able hands the matter has fallen-could decisively test my suggestion that plasticity is the main cause of the zero ascent after 120° C. Probably it has little or nothing to do with the ascent at the ordinary temperature. It is, however, known that fine threads of glass are undoubtedly plastic at the ordinary temperature. EDMUND J. MILLS.

Melrose, N.B., December 29, 1889.

Weismann's recent essays, is of a very different order from that forming the main position of the so-called Neo-Lamarckians in America. It is true that most American zoologists, somewhat upon Semper's lines, have supported the theory of the direct action of environment, always assuming, however, the question of transmission. But Cope, the able if somewhat extreme advocate of these views, with Hyatt, Ryder, Brooks, Dall, and others, holding that the survival of the fittest is now amply demonstrated, submit that, in our present need of an explanation of the origin of the fittest, the principle of selection is inadequate, and have brought forward and discussed the evidence for the inherited modifications produced by reactions in the organism itself-in other words, the indirect action of environment. The supposed arguments from pathology and mutilations have not been considered at all: these would involve the immediate inheritance of characters impressed upon the organism and not springing from internal reactions, and thus differ both in the element of time and in their essential principle from the above. As the selection principle is allowed all that Darwin claimed for it in his later writings, this school stands for Lamarckism plus-not versus-Darwinism, as Lankester has recently put it. There is naturally a diversity of opinion as to how far each of these principles is operative, not that they conflict.

The following views are adopted from those held by Cope and others, so far as they conform to my own observations and apply to the class of variations which come within the range of paleontological evidence. In the life of the individual, adaptation is increased by local and general metatrophic changes, of necessity correlated, which take place most rapidly in the regions of least perfect adaptation, since here the reactions are greatest; the main trend of variation is determined by the slow transmission, not of the full increase of adaptation, but of the disposition to adaptive atrophy or hypertrophy at certain points; the variations thus transmitted are accumulated by the selection of the individuals in which they are most marked and by the extinction of inadaptive varieties or species: selection is thus of the ensemble of new and modified characters. Finally, there is sufficient paleontological and morphological evidence that acquired characters, in the above limited sense, are transmitted.

In the present state of discussion, everything turns upon the last proposition. While we freely admit that transmission has been generally assumed, a mass of direct evidence for this assumption has nevertheless been accumulating, chiefly in the field of paleontology. This has evidently not reached Prof. Weismann, for no one could show a fairer controversial spirit, when he states repeatedly: "Not a single fact hitherto brought forward can be accepted as proof of the assumption." It is, of course, possible for a number of writers to fall together into a false line of reasoning from certain facts; it must, however, be pointed out that we are now deciding between two alternatives only, viz. pure selection, and selection plus transmission.

The distinctive feature of our rich palæontological evidence is that it covers the entire pedigree of variations: we are present not only at but before birth, so to speak. Among many examples, I shall select here only a single illustration from the mammalian series-the evolution of the molar teeth associated with the peculiar evolution of the feet in the horses. The feet, starting with plantigrade bear-like forms, present a continous series of readjustments of the twenty-six original elements to digitigradism which furnish proof sufficient to the Lamarckian. But, as selectionists would explain this complex development and reduction by panmixia and the selection of favourable fortuitous correlations of elements already present, the teeth render us more direct service in this discussion, since they furnish not only the most intricate correlations and readjustments, but the complete history of the addition

of a number of entirely new elements-the rise of useful structures from their minute embryonic, apparently useless, condition, the most vulnerable point in the pure selection theory. Here are opportunities we have never enjoyed before in the study of the variation problem.

The first undoubted ancestor of the horse is Hyracotherium; let us look back into the early history of its multicuspid upper molars, every step of which is now known. Upon the probability that mammalian teeth were developed from the reptilian type, Cope predicted in 1871 that the first accessory cusps would be found on the anterior and posterior slopes of a single cone, i.e. at the points of interference of an isognathous series in closing the jaws. Much later I showed that precisely this condition is filled in the unique molars of the Upper Triassic Dromotherium. These with the main cusp form the three elements of the tritubercular crown. Passing by several well-known stages, we reach one in which the heel of the lower molars intersects, and, by wearing, produces depressions in the transverse ridges of the upper molars. At these points are developed the intermediate tubercles which play so important a role in the history of the Ungulate molars. So, without a doubt, every one of the five main component cusps superadded to the original cones, is first prophesied by a point of extreme wear, replaced by a minute tubercle, and grows into a cusp. The most worn teeth, i.e. the first true molars, are those in which these processes take place most rapidly. We compare hundreds of specimens of related species; everywhere we find the same variations at the same stages, differing only in size, never in position. We extend the comparison to a widely separate phylum, and find the same pattern in a similar process of evolution. Excepting in two or three side lines the teeth of all the Mammalia have passed through closely parallel early stages of evolution, enabling us to formulate a law: The new main elements of the crown make their appearance at the first points of contact and chief points of wear of the teeth in preceding periods. Whatever may be true of spontaneous variations in other parts of the organism, these new cusps arise in the perfectly definite lines of growth. Now, upon the hypothesis that the modifications induced in the organism by use and disuse have no directive influence upon variations, all these instances of sequence must be considered coincidences. If there is no causal relationship, what other meaning can this sequence have? Even if useful new adjustments of elements already existing may arise independently of use, why should the origin of new elements conform to this law? Granting the possibility that the struggle for existence is so intense that a minute new cusp will be selected if it happens to arise at the right point, where are the non-selected new elements, the experimental failures of Nature? We do not find them. Palæontology has, indeed, nothing to say upon individual selection, but chapters upon unsuccessful species and genera. Here is a practical confirmation of many of the most forcible theoretical objections which have been urged against the selection theory.

Now, after observing these principles operating in the teeth, look at the question enlarged by the evolution of parallel species of the horse series in America and Europe, and add to the development of the teeth what is observed in progress in the feet. Here is the problem of correlation in a stronger form even than that presented by Spencer and Romanes. To vary the mode of statement, what must be assumed in the strict application of the selection theory? (a) that variations in the lower molars correlated with coincident variations of reversed patterns in the upper molars, these with metamorphoses in the premolars and pocketing of the incisor enamel; (b) all new elements and forms at first so minute as to be barely visible immediately selected and accumulated; (c) in the same individuals favourable variations in the proportions of the digits involving readjustments in the entire limbs and

skeleton, all coincident with those in the teeth; (d) finally, all the above new variations, correlations and readjustments, not found in the hereditary germ-plasm of one period, but arising fortuitously by the union of different strains, observed to occur simultaneously and to be selected at the same rate in the species of the Rocky Mountains, the Thames Valley, and Switzerland! These assumptions, if anything, are understated. Any one of them seems to introduce the element of the inconstant, whereas in the marvellous parallelism, even to minute teeth markings and osteological characters, in all the widely distributed forms between Hyracotherium and Equus, the most striking feature is the constant. Viewed as a whole, this evolution is one of uniform and uninterrupted progression, taking place simultaneously in all the details of structure over great areas. So nearly does race adaptation seem to conform to the laws of progressive adaptation in the individual, that, endowing the teeth with the power of immediate reactive growth like that of the skeleton, we can conceive the transformation of a single individual from the Eocene five-toed bunodont into the modern horse.

The special application of the Lamarckian theory to the evolution of the teeth is not without its difficulties, some of which have been pointed out to me by Mr. E. B. Poulton. To the objection that the teeth are formed before piercing the gum, and the wear produces a loss of tissue, it may be replied that it is not the growth, but the reaction which produces it, which is supposed to be transmitted. Again, this is said to prove too much; why is the growth of these cusps not continuous? This may be met in several ways: first, in the organism itself these reactions are least in the best adapted structures, a proposition which is more readily demonstrated in the feet than in the teeth-moreover, since the resulting growth never exceeds the uses of the individual, there is a natural limit to its transmission; secondly, the growth of the molars is limited by the nutritive supply-we observe one tooth or part growing at the expense of another; third, in some phyla we do observe growth which appears to lead to inadaptation and is followed by extinction. In one instance we observe the recession of one cusp taking place pari passu with the development of the one opposed to it. These and many more general objections may be removed later, but they are of such force that, even granting our own premises, we cannot now claim to offer a perfectly satisfactory explanation of all the facts.

The evidence in this field for, is still much stronger than that against, this theory. To sum up, the new variations in the skeleton and teeth of the fossil series are observed to have a definite direction; in seeking an explanation of this direction, we observe that it universally conforms to the reactions produced in the individual by the laws of growth; we infer that these reactions are transmitted. If the individual is the mere pendent of a chain (Galton), or upshoot from the continuous root of ancestral plasm (Weismann), we are left at present with no explanation of this well-observed definite direction. But how can this transmission take place? If, from the evident necessity of a working theory of heredity, the onus probandi falls upon the Lamarckian-if it be demonstrated that this transmission does not take place-then we are driven to the necessity of postulating some as yet unknown factor in evolution to explain these purposive or directive laws in variation, for, in this field at least, the old view of the random introduction and selection of new characters must be abandoned, not only upon theoretical grounds, but upon actual observation.

Reading between the lines of Weismann's deeply interesting essays, it is evident that he himself is coming to this conclusion. HENRY FAIRFIELD OSBORN. Princeton College, August 23.