THE LIFE CYCLE 261 



the young animals get further and further along in the 

 course of their development, they become more and more 

 different until each finally reaches its fully developed ma- 

 ture form, showing all the great structural differences be- 

 tween the star-fish and the dove, the beetle and the horse. 

 That is, all animals begin development alike, but gradually 

 diverge from each other during the course of development. 

 There are some extremely interesting and significant 

 things about this divergence to which attention should be 

 given. While all animals are alike structurally* at the 

 beginning of development, so far as we can see, they do not 

 all differ at the time of the first divergence in development. 

 This first divergence is only to be noted between two kinds 

 of animals which belong to different great groups or classes. 

 But two animals of different kinds, both belonging to some 

 one great group, do not show differences until later'ln their 

 development. This can best be understood by an example. 

 All the butterflies and beetles and grasshoppers and flies 

 belong to the great group of animals called Insecta, or in- 

 sects. There -are many different kinds of insects, and these 

 kinds can be arranged in subordinate groups, such as the 

 Diptera, or flies, the Lepidoptera, or butterflies and moths, 

 and so on. But all have certain structural characteristics 

 in common, so that they are comprised in one great group 

 or class the Insecta. Another great group of animals is 

 known as the Vertebrata, or back-boned animals. The class 

 Vertebrata includes the fishes, the batrachians, the reptiles, 

 the birds, and the mammals, each composing a subordinate 

 group, but all characterized by the possession of a back- 



* They are alike structurally, when we consider the cell as the unit 

 of animal structure. That the egg cells of different animals may dif- 

 fer in their fine or ultimate structure, seems certain. For each one of 

 these egg cells is destined to become some one kind of animal, and no 

 other ; each is, indeed, an individual in simplest, least developed con- 

 dition of some one kind of animal, and we must believe that difference 

 in kind of animals depends upon difference in structure in the egg itself. 



