frequently a structure that is anatomically the same. The bones of the human arm and hand have their homologues in the legs and feet of all quadrupeds, in the wings of all birds, and in the pectoral fin of the fish, and the flipper of the seal. The hoof of the horse is on his middle finger; the other phalanges are rudimental, though all present. The carpal and metacarpal bones are also partially represented in the legs of the horse; so also is the radius, though these bones are now rudimentary and useless. Among other rudimentary structures may be mentioned the foetal teeth of whales and of the front part of the jaw of ruminant quadrupeds. "These foetal structures are minute in size, and never cut the gum; but are reabsorbed without ever coming into use, while no other teeth succeed them or represent them in the adult condition of those animals. The mammary glands of all male beasts constitute another example, as also does the wing of the apteryx, -a New Zealand bird utterly incapable of flight, and with the wing in a quite rudimentary condition (whence the name of the animal). Yet this rudimentary wing contains bones which are miniature representatives of the ordinary wing-bones of birds of flight." Is there in all this any meaning which the human mind can interpret? Do these facts have any natural correlation to those innate tendencies of the mind on which beliefs are based? Is their glimmer of light in any degree trustworthy, and if so, to what degree? Or are they altogether like Willo'the Wisps going before us but to deceive? Mr. Darwin's comparison has the merit of being clear, if not cogent. 66 Rudimentary organs may be compared with the letters in a word, still retained in the spelling, but become useless in the pronunciation, but which serve as a clue for its derivation. On the view of descent with modification, we may conclude that the existence of organs in a rudimentary, imperfect, and useless condition, or quite aborted, far from presenting a strange difficulty, as they avowedly do on the old doctrine of creation, might even have been anticipated in accordance with the views here explained." 1 See Mivart, Genesis of Species, pp. 7, 155-187. XI. EMBRYOLOGY. Another class of facts presenting peculiar difficulties to the ordinary hypothesis of special creation, relates to the process of development through which the young animal passes in its embryonic condition. We quote again from the elementary work on Zoölogy by Professor Agassiz. "As a general result of the observations which have been made up to this time [1855] on the embryology of the various classes of the animal kingdom, especially of the Vertebrates, it may be said, that the organs of the body are successively formed in the order of their organic importance, the most essential being always the earliest to appear. In accordance with this law, the organs of vegetative life, the intestines and their appurtenances, make their appearance subsequently to those of animal life, such as the nervous system, the skeleton, etc.; and these, in turn, are preceded by the more general phenomena belonging to the animal as such... Hence the embryos of different animals resemble each other more strongly when examined in the earlier stages of their growth. We have already stated that, during almost the whole period of embryonic life, the young fish and the young frog scarcely differ at all; so it is also with the young snake compared with the embryo bird." 1 2 "This similarity of members of the same great class, in their embryonic condition, the embryo, for instance, of a mammal, bird, reptile, and fish being barely distinguishable," is pronounced by Darwin "the most wonderful fact in the whole round of natural history." That the embryos of the higher vertebrates should in their development pass through all the stages of the lower orders of their class, taking upon them at successive stages the peculiarities that characterize the order, the family, the genus, the species, and the individual; that this order coincides with the distribution of species in time; and that rudimentary organs are often developed at particular stages of the growth, and then partially or wholly re-absorbed, are certainly coincidences which it is hard to accept as accidental or meaningless. But on the theory of a common descent with modifications, all these facts come in harmoniously, this element of descent 1 Principles of Zoology, p. 153. 2 Animals and Plants under Domestication, Vol. i. p. 24. being the hidden bond of connection which naturalists, in their efforts at classification, have been seeking, under the term of the natural system.1 XII. ANALOGOUS VARIATION. An argument is also drawn from the facts of analogous variation. For instance, distinct breeds, like those of the domestic pigeon, which are now very unlike, tend to vary in a similar manner, resembling one wild species from which they are supposed to have descended. The slaty-blue color and the black bars across the wings of the original rock pigeon are occasionally assumed by individuals of all the varieties, though when kept pure they usually breed true and have no trace of those colors. When, however, different breeds are crossed, the tendency of these black bars and this blue color to appear is greatly increased, and the peculiarities of the crossed birds disappear. Similar facts afford proof of the affinity of the horse and the ass to the zebra. By a wide induction, Darwin has shown that the appearance of the stripes which characterize the zebra are sometimes seen on every variety both of the horse and of the ass. And furthermore, that the mule, which is a cross between the horse and the ass, is much more likely than either to display those characteristic stripes, especially when young. Upon this Darwin remarks: 2 "He who believes that each equine species was independently created, will, I presume, assert that each species has been created with a tendency to vary, both under nature and under domestication, in this particular manner, so as often to become striped like other species of the genus; and that each has been created with a strong tendency, when crossed with species inhabiting distant quarters of the world, to produce hybrids resembling in their stripes, not their own parents, but other species of the genus. To admit this view is, as it seems to me, to reject a real for an unreal, or at least for an unknown, cause. It makes the works of God a mere mockery and deception. I would almost as soon believe with the old and ignorant cosmologists that fossil shells had never lived, but had been created in stone, so as to mock the shells living on the sea-shore." 1 See Origin of Species, pp. 381, 396 and 403. 2 Origin of Species, p. 130. XIII. SUMMARY OF FACTS. Before proceeding to an explanation of these phenomena, we will briefly recapitulate. If in the animal kingdom we take one of the departments, Vertebrata, for instance, we find that all the individuals are characterized by certain fundamental likenesses, and are distinguished by varying degrees of unlikeness. Upon the bond of the similarity characterizing the grand division, the differences are superimposed which designate the more specific stages of our advancement in classification. There is a natural order of classification, so that starting along certain lines of divergence, and passing through more and more restricted clusters of likenesses, we reach a system of species and varieties and individuals, branching off from a common point, in which there is no intermingling and little ground for confusion. Theories of evolution have in their favor the analogies of the known mode of the production of individuals. So far as we know, individuals are born and developed; not produced by a direct act of creation, or by spontaneous generation. "Every life is from an egg.' So constant is this law that the supposed production of a living thing without a cell for its origin is strong proof either of the incompetence of the observer's method or of the imperfection of his instruments. The natural system of classification corresponds in general with the embryonic development of each individual. The more generic characters of the animal appear first in the developing embryo. The specific characters are superinduced from time to time, as the period of birth approaches, or, indeed, long afterwards, in the post-natal development. In the distribution of animals in time the same order of development is observable. The earlier forms of life that are studied in fossil remains are, as a rule, more generalized in their structure than the later forms. Classes of animals, like birds, reptiles, and fishes, were not so clearly distinguishable in the early Tertiary and in the Maesozoic times as now. Again, in space animals and plants are separated by natural barriers. The farther you recede from the continental hemisphere of the earth the more diverse the existing forms of life are from each other, and the nearer they resemble the more generalized forms of past time. Also the forms of life on islands are, as a rule, conformed not so much to the existing conditions of soil and climate as to the type of animal life on the nearest continental area. XIV. PROPER TO SPECULATE UPON THE PROBLEM. The foregoing are the more important of the facts that press upon us for an explanation. It is not in accordance with what we specially value in the modern habits of thought to cut the Gordian knot with the assertion, "so God has made it," and set that up as the Ultima Thule of our investigation. Such a course would be suicidal to all scientific thought, and would endanger the rational foundation upon which our proof even of revelation rests. It is superstition, and not reverence, that leads us to avoid the questions concerning the order and mode of divine operations. It is a principle never to be forgotten in any department of study that we are to press known secondary causes as far as they will go in explanation of facts. We are not to resort to an unknown cause for explanation of phenomena till the power of known causes has been exhausted. If we cease to observe this rule there is an end to all science and all sound sense. In viewing the complicated movements of the heavenly bodies, it would relieve us from much labor, if we should simply register the phenomena, and attribute them directly to the divine activity. Newton, however, was not satisfied till he had interpreted the laws under which these movements proceed. He believed that in the peculiarities of planetary movements God permitted us to read the method of his operations. By a most successful application of the law of parsimony all that variety of movement in cycle and epicycle was traced to the effects of two forces, centripetal and centrifugal; the one constant, the other varying as the square of the distance between the attracting objects. |