364 DEVELOPMENT AND EMBKYOLOGY. [CHAP. XIV. 



It has already been stated that various parts in the same indi- 

 vidual which are exactly alike during an early embryonic period, 

 become widely different and serve for widely different purposes in 

 the adult state. So again it has been shown that generally the 

 embryos of the most distinct species belonging to the same class 

 are closely similar, but become, when fully developed, widely dis- 

 similar. A better proof of this latter fact cannot be given than the 

 statement of Von Baer that " 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 tho 

 " 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, 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 birds, no less than the hands and feet of man, all 

 "arise from the same fundamental form." The larvae of most crus- 

 taceans, at corresponding stages of development, closely resemble 

 each other, however different the adults may become ; and so it is 

 with very many other animals. A trace of the law of embryonic 

 resemblance occasionally lasts till a rather late age : thus birds of 

 the same genus, and of allied genera, often resemble each other in 

 their immature plumage ; as we see in the spotted feathers in the 

 young of the thrush group. In the cat tribe, most of the species 

 when adult are striped or spotted in lines ; and stripes or spots can 

 be plainly distinguished in the whelp of the lion and the puma. 

 We occasionally though rarely see something of the same kind in 

 plants ; thus the first leaves of the ulex or furze, and the first leaves 

 of the phyllodineous acacias, are pinnate or divided like the 

 ordinary leaves of the leguminosse. 



The points of structure, in which the embryos of widely different 

 animals within the same class resemble each other, often have no 

 direct relation to their conditions of existence. We cannot, for 

 instance, suppose that in the embryos of the vertebrata the peculiar 

 loop-like courses of the arteries near the branchial slits are related 

 to similar conditions, in the young mammal which is nourished 

 in the womb of its mother, in the egg of the bird which is hatched 

 in a nest, and in the spawn of a frog under water. We have no 

 more reason to believe in such a relation, than we have to believe 

 that the similar bones in the hand of a man, wing of a bat, and fia 

 of a porpoise, are related to similar conditions of life. No one 



