I do not know that Professor Weismann has anywhere distinctly stated what he conceives to be the relation of body-plasm and germ-plasm in the protozoa. Are the two as yet undifferentiated? This can hardly be so, seeing the fundamental distinction he draws between them. Is it the germ-plasm or the body-plasm that is influenced by external stresses? If the former, does it transfer its influence to the body-plasm during the life of the individual? If the latter, then the body-plasm must either directly influence the germ-plasm in unicellular organisms (it would seem that, according to Professor Weismann, it cannot do so in the metazoa), or the changed body-plasm, which shares in the fission of the protozoon, must participate in that so-called immortality which is often said to be the special prerogative of germinal matter.

These, however, are matters for Professor Weismann and his followers to settle. I regard the sharp distinction between body-plasm and germ-plasm as an interesting biological myth. For me, it is sufficient that the protoplasm of the protozoon is modified, and the modification. handed on in fission. And it is clear that Professor Weismann is correct in saying that the commixture or combination of characters takes its origin among the protozoa. If the unicellular individuals are differently modified, however slightly, then, whenever conjugation occurs between two such individuals, there will be a commingling or combination of the different characters. The transmissible influence of the environment, however, ceases when the metazoon status is reached, and special cells are set apart for reproductive purposes-ceases, that is to say, in so far as the influence on the body is concerned. There may, of course, be still some direct* influence on the germinal cells themselves. Except for this further influence, the metazoon starts with the stock of variations

Darwin spoke of changed conditions acting "directly on the organization or indirectly through the reproductive system." Now, since Professor Weismann has taught us to reconsider these questions, we speak of such conditions as acting directly on the germ or indirectly through the body. The germ is no longer subordinate to the body, but the body to the germ.

acquired by that particular group of protozoa-whatever it may be from which it originated. All future variations in even the highest metazoa arise from these.

Now, it is obvious that no mere commingling and rearrangement of protozoan characters could conceivably give rise to the indefinitely more complex metazoan characters. But if there be a combination and recombination of these elements in ever-varying groups, the possibilities are no longer limited. Let us suppose that three simple protozoan characters were acquired. The mere commixture of these three could not give much scope for further variation. It would be like mixing carbon, oxygen, and hydrogen in varying proportions. But let them in some way combine, and you have, perhaps, such varied possibilities as are open to chemical combinations of oxygen, hydrogen, and carbon, whose name is legion, but whose character is determined by the laws of chemical combination.

Summing up now the origin of variations, apart from those which are merely individual, on the hypothesis that particular modifications of the body-cells cannot be transmitted to the germ-cells, we have

1. In protozoa, the direct influence of the environment and the induced development of faculty.

2. In metazoa

(a) Some direct and merely general influence of the environment on the germ, including under the term "environment" the nutrition, etc., furnished by the body.

(b) The combination and recombination of elementary protoplasmic faculties (specific molecular groupings) acquired by the protozoa.

(c) Influences on the germ, the nature of which is at present unknown.

We may now pass on to consider the position of those who give an affirmative answer to the question-Can the body affect the germ? Two things are here required. First, definite evidence of the fact that the body does so

affect the germ; i.e. that acquired characters are inherited. Secondly, some answer to the question-How are the bodycells able to transmit their modifications to the germ-cells? We will take the latter first, assuming the former point to be admitted.

Let us clearly understand the question. An individual, in the course of its life, has some part of the epidermis, or skin, thickened by mechanical stresses, or some group of muscles strengthened by use, or the activity of certain brain-cells quickened by exercise: how are the special modifications of these cells, here, there, or elsewhere in the body, communicated to the germ, so that its products are similarly modified in the offspring? The following are some of the hypotheses which have been suggested:

(a) Darwin's pangenesis.

(b) Haeckel's perigenesis; Spencer's physiological units. (c) The conversion of germ-plasm into body-plasm, and its return to the condition of germ-plasm (Nägeli). (d) The unity of the organism.

(a) Concerning pangenesis, nothing need be added to what has already been said. Although, as we have seen, it has been adopted with modifications by Professor Brooks; although Mr. Francis Galton, a thinker of rare ability and a pioneer in these matters, while contending for continuity, admitted a little dose of pangenesis; although De Vries has recently renewed the attempt to combine continuity and a modified pangenesis; this hypothesis does not now meet with any wide acceptance.

(b) With the pamphlet in which Professor Haeckel brought forward his hypothesis termed the perigenesis of the plastidule, I cannot claim first-hand acquaintance. According to Professor Ray Lankester, who gave some account of it in Nature,* protoplasm is regarded by Haeckel as consisting of certain organic molecules called plastidules. These plastidules are possessed of special undulatory movements, or vibrations. They are liable to have their undulations affected by every external force, and, once * July 15, 1876. Since reprinted in "The Advancement of Science," p. 273.

modified, the movement does not return to its pristine condition. By assimilation, they continually increase to a certain size and then divide, and thus perpetuate in the undulatory movement of successive generations the impressions or resultants due to the action of external agencies on the individual plastidules. On this view, then, the form and structure of the organism are due to the special mode of vibration of the constituent plastidules. This vibration is affected by external forces. The modified vibration is transmitted to the plastidules by the germ, which, therefore, produce a similarly modified organism. As Mr. J. A. Thomson says, "In metaphorical language, the molecules remember or persist in the rhythmic dance which they have learned."

Darwin's hypothesis was frankly and simply organicthe gemmules are little germs. This of Professor Haeckel tries to go deeper, and to explain organic phenomena in terms of molecular motion. Mr. Herbert Spencer long ago suggested that, just as molecules are built up, through polarity, into crystals, so physiological units are built up, under the laws of organic growth, into definite and special organic forms. Both views involve special units. With Mr. Herbert Spencer, their "polarity" is the main feature ; with Professor Haeckel, their "undulatory movements." According to Mr. Spencer, "if the structure of an organism is modified by modified function, it will impress some corresponding modification on the structures and polarities of its units." * According to Professor Haeckel, the vibrations of the plastidules are permanently affected by external forces. In either case, an explanation is sought in terms of molecular science, or rather, perhaps, on molecular analogies. So far good. Such "explanation," if hypothetical, may be suggestive. It may well be that the possibilities of fruitful advance will be found on these lines.

But though, as general theories, these suggestions may be valuable, they do not help us much in the comprehension of our special point. To talk vaguely about "undula*Herbert Sper cer, "Principles of Biology," vol. i. p. 256.

tory movements or "polarities" does not enable us to comprehend with any definiteness how this particular modification of these particular nerve-cells is so conveyed to the germ that it shall produce an organism with analogous nerve-cells modified in this particular way.

(c) The hypothesis that the germ-plasm may be converted into body-plasm, which, on its return again to the condition of germ-plasm, may retain some of the modifications it received as body-plasm, seems to be negatived, so far as most animals are concerned, by the facts of embryology and development. The distinction of germplasm and body-plasm I hold to be mythical. And there is no evidence that cells specially differentiated along certain lines can become undifferentiated again, and then contribute to the formation of ova or sperms. From the view-point of cell-differentiation, which seems to me the most tenable position, there does not seem any evidence for, or any probability of, the occurrence of any roundabout mode of development of the germinal cells which could enable them to pick up acquired characters en route.

(d) We come now to the contention that the organism, being one and continuous, if any member suffers, the germ suffers with it. The organs of the body are not isolated or insulated; the blood is a common medium; the nerves ramify everywhere; the various parts are mutually dependent may we not, therefore, legitimately suppose that long-continued modification of structure or faculty would soak through the organism so completely as eventually to modify the germ? The possibility may fairly be admitted. But how is the influence of the body brought to bear on the germ? The common medium of the blood, protoplasmic continuity, the influence of the products of chemical or organic change, these are well enough as vague suggestions. But how do they produce their effects? Once more, how is this increased power in that biceps muscle of the oarsman able to impress itself upon the sperms or the ova? No definite answer can be given.

We are obliged to confess, then, that no definite and

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