DEVELOPMENT AND EMBRYOLOGY. 4^9 



cies belonging to the same class are closely similar, but 

 become, when fully developer], widely dissimilar. A better 

 proof of this latter fact cannot be given than the statement 

 by Von Baer that " the embryos of mammalia, of birds, 

 lizards and snakes, probably also of chelonia, are in the 

 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 com- 

 plete is the similarly 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 

 crustaceans, at corresponding stages of development, closely 

 resemble each other, however diiferent the adults may be- 

 come ; 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 plum- 

 age ; 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 legumi- 

 nosse. 



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 



