388 Development and Embryology. Chap. xiv. 



" 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, 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 develop- 

 " ment we should learn nothing, for the feet of lizards and mam- 

 " mals, the wings and feet of birds, no less than the hands and feet 

 " of man, all arise from the same fundamental form." The larvaa 

 of most crustaceans, 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 leguminosas. 



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 fin 

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

 supposes that the stripes on the whelp of a lion, or the spots on 

 the young blackbird, are of any use to these animals. 



The case, however, is different when an animal during any part 

 of its embryonic career is active, and has to provide for itself. The 

 period of activity may come on earlier or later in life ; but whenever 



