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tozoa continue to closely resemble one another at the next succeeding step. This step forms what Professor Haeckel calls the “gastrula-stage.”
" It seems perfectly certain, then, that, if the mulberrystage constitutes a first landmark in the development of the animal kingdom at large, no less does the gastrulastage form a second resting-place in the track of life: since, as Haeckel and other embryologists have shown us, the gastrula-stage of development (with its primitive
mouth [m], body-cavity or stomach [i], and double layers [pc and en] occurs equally in the zoophyte (5) and worm (1), is as typical of the star-fish (2) as of the crustacean (3), and aids as materially in the formation of the snail (4) as in the development of the vertebrate (6). After its gastrula-stage, each animal form may be said to assume the special features of the group to which is belongs.” 1
Within the limits of each sub-kingdom, the process of embryonic growth and development, onward from the gastrula-stage, is not less striking in its uniformity. To give even a hasty account of the process would involve many pages of description, and it will here suffice to state only a few interesting facts in regard to the strong likeness that prevails among the embryos of vertebrates. Says Von Baer, the father of embryology,
“ The embryos of mammalia, of birds, lizards, and snakes, probably also of chelonia,” are in their earliest states exceedingly like one another, both as a whole, and in the mode of development of their parts; so much so, in fact, that we can often distinguish the embryos only by their size. In my possession are two little embryos in spirit [alcohol], whose names I have omitted to attach ; and at present I am quite unable to say to what class they belong. They may be lizards, or small birds, or very young mammalia, so complete is the similarity in the mode of formation of the head and trunk in these animals. The extremities, however, are still absent in these embryos. But, even if they had existed, in the earliest stage of their development we should learn nothing; for the feet of lizards and mammals, the wings and feet of
Wilson's Chapters on Evolution, pp. 186, 187. 2 Turtles and tortoises.
birds, no less than the hands and feet of man, all arise from the same fundamental form.” 1
And not only is there noticeable this wonderful resemblance among the embryos of existing animals of the same sub-kingdom; but it also seems probable, according to Professor Louis Agassiz, — and it must be remembered that there have been few opponents of the development theory so illustrious as Agassiz, — that all embryos contain reminiscences of, or points of resemblance to, the full-grown, pre-existing animals of the same group. For example, if we find (as is the case), that, in the embryo of birds, the fore-limb or wing is for a long while much like the foot, this would be, according to Professor Agassiz's theory, a reason for at least supposing that the earliest birds, of previous geological ages, had fore-limbs more like feet than like the wings of birds of the present day.
In many cases, the study of fossil remains has proved beyond a doubt that the view just stated is a true one. But, in adopting this view, how difficult it is to avoid coming to the conclusion that existing animals at some period of their history resemble the most nearly allied among those which preceded them, simply because the present kinds are descended from those earlier ones, or at any rate both are descended from common ancestors! And this ready explanation clears up very many dificulties in the same way that one is relieved to find that a strong resemblance between two people arises, very naturally, from the fact (of which we were not at first aware) that they are relatives.
1 Quoted by Darwin, Origin of Species, p. 387.
Professor Haeckel has strongly emphasized the doctrine, that, in all cases, the history of the development of each individual portrays the history of the development of its tribe from its origin, back amid the imperfectly recorded events of the geological past, till now. According to this view, then, the presence of the gastrulastage in the development of individuals in so many sub-kingdoms suggests that all the higher of those sub-kingdoms are descended from a lower one. If we look for a common ancestry for plants and animals, the almost indistinguishable closeness of resemblance between the lowest forms of either kingdom offers a substantial basis for such an hypothesis. So little has as yet been done toward a minute application of Haeckel's law of embryology to the life-history of plants, that it will not be possible to trace the subject out in the present chapter. Professor William Trelease has kindly furnished the authors with a brief sketch of the matter, which is necessarily so much more technical than the main portion of this book, that it has seemed best to insert it in the Appendix.1
One subject more remains to be treated in connection with embryology; that is, the existence of rudimentary or abortive organs. It has long been known to naturalists, that many animals and plants are possessed of members, parts, or organs of which they make no use; although such organs usually have their counterparts in useful structures of the same kind, - either in individuals of the same species but the opposite sex, or in other related genera, families, or orders. How such a change as the partial loss of an organ might prove beneficial to an animal has already been alluded to in the case of the tu co-tuco, which seems to be gradually losing its eyesight, and will be better adapted to its underground mode of lifc wheż relieved of the inflammation of the eyes with which it is now frequently troubled. Snakes and many
1 See Appendix.
2 Such members of an animal or plant as are imperfectly developed, and of no use.