86 ANIMAL LIFE 



bone, or, more accurately speaking, of a notochord, a back- 

 bone-like structure. Now, an insect and a vertebrate di- 

 verge very soon in their development from each other ; but 

 two insects, such as a beetle and a honey-bee, or any two 

 vertebrates, such as a frog and a pigeon, do not diverge 

 from each other so soon. That is, all vertebrate animals 

 diverge in one direction from the other great groups, but 

 all the members of the great group keep together for some 

 time longer. Then the subordinate groups of the Verte- 

 brata, such as the fishes, the birds, and the others diverge, 

 and still later the different kinds of animals in each of 

 these groups diverge from each other. In the illustration 

 (Fig. 41) on the opposite page will be seen pictures of the 

 embryos of various vertebrate animals shown as they appear 

 at different stages or times in the course of development. 

 The embryos of a fish, a salamander, a tortoise, a bird, and 

 a mammal, representing the five principal groups of the 

 Vertebrata, are shown. In the upper row the embryos are 

 in the earliest of all the stages figured, and they are very 

 much alike. There are no distinctive characteristics of 

 fish or bird. Yet there are distinctive characteristics of 

 the great class Vertebrata. Any of these embryos could 

 readily be distinguished from an embryonic insect or worm 

 or sea-urchin. In the second row there is beginning to be 

 manifest a divergence among the different embryos, al- 

 though it would still be a difficult matter to distinguish 

 certainly which was the young fish and which the young 

 salamander, or which the young tortoise and which the 

 young bird. In the bottom row, showing the animals in a 

 later stage of development, the divergence has proceeded 

 so far that it is now plain which is a fish, which batrachian, 

 which reptile, which bird, and which mammal. 



54. The laws or general facts of development. That the 

 course of development of any animal from its beginning to 

 fully developed adult form is fixed and certain is readily 

 seen. Every rabbit develops in the same way ; every grass- 



