GENERATION, SEX AND ONTOGENY 231 



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 backbone, 

 or, more accurately speaking, of a notochord, a backbonelike 

 structure. Now, an insect and a vertebrate diverge very 

 soon in their development from each other; but two insects, 

 such as a beetle and a honeybee, 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 Vertebrata, 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. 



That the course of development of any animal from its 

 beginning to fully developed adult form is in all its essentials 

 fixed and certain is readily seen. All rabbits develop in 

 the same way; every grasshopper goes through the same de- 

 velopmental changes from single egg cell to the full-grown, 

 active hopper as every other grasshopper of the same kind 

 that is, development takes place according to certain natural 

 laws: the laws of animal development. These laws may be 

 roughly stated as follows: All many-celled animals begin life 

 as a single cell, the fertilized egg cell; each animal goes through 

 a certain orderly series of developmental changes which, ac- 

 companied by growth, leads the animal to change from single 

 cell to the many-celled, complex form characteristic of the 

 species to which, the animal belongs; this development is from 

 simple to complex structural condition; the development is 

 the same for all individuals of one species. While all animals 

 begin development similarly, the course of development in 

 the different groups soon diverges, the divergence being of the 

 nature of a branching, like that shown in the growth of a tree. 

 In the free tips of the smallest branches we have represented 

 the various species of animals in their fully developed con- 

 dition, all standing more or less clearly apart from each other. 

 But in tracing back the development of any kind of animal 

 we soon come to a point where it very much resembles or 

 becomes apparently identical with the development of some 

 other kind of animal, and, in addition, the stages passed through 

 in the developmental course may very much resemble the 



