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Show H8 DARWINISM CHAP. The Emb1·yonic Development of 111an cmd othe1· Mammalia. The pro(J'ressive development of any vertebrate from the ovum or ~inutc embryonic egg affords one of the most marvellous chapters in Natural History. "\V e ~ee the contents of the ovum undergoing numerous defimte . changes, its interior dividin(J' and subdividing till it con. 1sts of a mass of cells then ab (J'roove appears marking out the median line or vert~bral colubmn of the future animal, and thereafter are slowly developed the various ~sscntial organs of the body. After describing in some deta1l what takes place in the case of the ovum of the dog, Professor Huxley continues: "The hi tory of the development of any other vertebrate animal, liza.rd, snake, frog, or fish tells th_e same story. There is always to begin with, an egg havmg the same essential structure as that of the dog ; the yolk of that egg undergoes division or segmentati?n, as it ~s callecl, the ultimate products of that segmentatiOn cons.t1tute the buildin(J' materials for the body of the young ammal; and this is built up round a primitive groove, in the ~oor of :vhi~h a notochord is developed. Furthermore, there Is a penod m which the young of all these an~mals resem?le one another, not merely in outward form, but m all essentmls of_ struc~nre, so closely, that the differences between then: are mcon ·ulerable while in their subsequent course they diverge more and. mor~ widely from one another. And it is a general ~aw that the more closely any animals resemble one another m adult structure, the larger and the more intima.tcly do their em hryos re emble one another; so that, for example, the embryos of a snake and of a lizard remain like one another longer than do those of a snake and a bird; and the embryos of a dog n.ml of a cat remain like one another for a far longer period than do those of a do()' and a bird, or of a dog and an opossum, or even than those bo f a, dog and a mon l rey. "1 We thus see that the study of development affords a tc~t of affinity in animc.tls that are externa.lly .very n:uch unlike each other; and we naturally ask how this apphes to mn.n. Is he developed in a different way from other m~n:mals, a~ we should certainly expect if he has had a d1stmct a11U 1 .frlan's Plctee in NaltG?·e, p. 64. XV DARWINISM APPLIED TO MAN 449 altogether different origin ? "The reply," says Professor Huxley, "is not doubtful for a moment. "\Vithout question, the mode of origin and the early stages of the development of man are identical with those of the animals immediatel below him in the scale." And again he tells us: "It is very long before the body of the young human being can be readily discriminated. from that of the young puppy ; but at a tolerably early period the two become distingui ·hable by th different forms of their adjuncts, the yelk-sac and the al lantois; '' and after describing these difference he continues: "But exactly in those respects in which the developing man differs from the dog, he resembles the ape .... So that it is only quite in the latter stages of development that the young human being presents marked differences from the young ape, while the latter departs as much from the dog in its development as the man docs. Startling as this last assertion may appear to be, it is demonstrably true, and it alone appears to me sufficient to place beyond all doubt the structuml unity of man with the rest of the animal world, and more particularly and closely with the apes."I A few of the curious details in which man pa. ses through stages common to the lower animals may be mentioned. At one stage the os coccyx projects like a true tail, extending considerably beyond the rudimentary legs. In the seventh month the convolutions of the brain resemble those of an adult baboon. The great toe, so characteristic of man, forming the fulcrum which most assists him in standing erect, in an early stage of the embryo is much . horter than the other toes, and instead of being parallel with them, projects at an angle from the side of the foot, thus corresponding with its permanent condition in the quadrumana. Numm·ous other examples might be quoted, all illustrating the same general law. Diseases Common to Man and the Lower Animals. Though the fact is so well known, it is certainly one of profound significance that many animal diseases can be communicated t9 man, since it shows similarity, if not identity, in 1 lvlan's Place in Natu1·e, p. 67. See Figs. of Embryos of Man and Dog in Darwin's Descent of lvlan, p. 10. 2G |