It has been stated that the palæontological record is quite incomplete, owing to many facts, some of which have been mentioned; fortunately, the history of the development of the organic individual, or ontogeny, comes in to fill up many deficiencies.
Ontogeny is a repetition of the principal forms through which the respective individuals have passed from the beginning of their tribe, and its great advantage is that it reveals a field of information which it was impossible for the rocks to retain; for the petrification of the ancient ancestors of all the different animal and vegetable species, which were soft, tender bodies, was not possible.
The annexed plate illustrates the dog, rabbit, and man in their first stages of development. Illustrations of a fish, an amphibious animal, a reptile, a bird, or any mammal, could also be given; for all vertebrate animals of the most different classes, in their early stages of development, cannot be distinguished, and the nearer the animal approaches man in the ascending scale, the longer does this similarity continue to exist—when reptiles and birds are distinctly different from mammals, the dog and the man are almost identical.
The gill-arches of the fish exist in man, in dogs, in fowls, in reptiles, and in other vertebrate animals during the first stages of their development. Man also possesses, in his first stages, a real tail, as well as his nearest kindred—the tailless apes (orang-outang, chimpanzee, gorilla), and vertebrate animals in general. The tail, as has been stated, man still retains, though hidden as a rudiment.
| Fig. I. | Fig. IV. | Fig. VII. | |
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| Fig. II. | Fig. V. | Fig. VIII. | |
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| Fig. III. | Fig. VI. | Fig. IX. | |
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Fig. I.—Human Embryo.—Ecker.
Fig. II.—Embryo of Dog.—Bischoff.
Fig. III.—Dog Embryo.—Huxley.
Figs. IV, V, and VI.—Embryo of Rabbit in three stages of development.—Haeckel.
Figs. VII, VIII, and IX.—Embryo of Man in three stages of development.—Haeckel. v, fore brain; z, twix brain; m, middle brain; h, hind brain; n, after brain; r, spinal marrow; e, nose; a, eye; o, ear; k, gillarches; g, heart; w, vertebral column; f, fore limbs; b, hind limbs; s, tail.
"Man presents in his earliest stages of embryonic growth, a skeleton of cartilage, like that of the lamprey; also, five origins of the aorta and five slits on the neck, like the lamprey and the shark. Later, he has but four aortic origins, and a heart now divided into two chambers, like bony fishes; the optic lobes of his brain also having a very fish-like predominance in size. Three chambers of the heart and three aortic origins follow, presenting a condition permanent in the batrachia; then two origins with enlarged hemispheres of the brain, as in reptiles. Four heart chambers and one aortic root on each side, with slight development of the cerebellum, agree with the characters of the crocodiles, and immediately present the special mammalian conditions, single aortic root, and the full development of the cerebellum. Later comes that of the cerebrum, also in its higher mammalian or human traits." At no time in the development of the egg, save at the start, do the embryos of the various vertebra assume the exact or entire characteristics of one another, but they assimilate so closely that it requires the eye of the expert to distinguish them; and, as has already been stated, the more closely an animal resembles another, the longer and the more intimately do their embryos resemble one another; so that, for example, the embryo of the snake and of a lizard remain like one another longer than do those of a snake and of a bird; and the embryo of a dog and of a cat remain like one another for a far longer period than do those of a dog and a bird, or a dog and an opossum, or even those of a dog and a monkey.
Surely it must be admitted that the short brief history given by the development of the egg, is far more wonderful than phylogeny or the long and slow history of the development of the tribe, which has taken thousands of years. Compare this time with the time required for the development of the smallest mammals—the harvest mice which develops in three weeks, or the smallest of all birds, the humming-bird, which quits the egg on the twelfth day, or with man who passes through the whole course of his development in forty weeks, or with the rhinoceros who requires 1½ years, or the elephant who requires ninety weeks. How insignificant are these various periods to the long period originally required; yet in these short periods the whole phylogeny is run through in the ontogeny or the history of the development of the egg.





