GENERATION, SEX AND ONTOGENY 231 



Vertebrata incliulcs tlio fislios, tlic ])atracliiaiis, the reptiles, 

 the birds, and the maininals, vivAx coinposin^ a subordinate 

 group, but all charactorized l)y the jwssession of a backbone, 

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

 structure. Now, an insect and a vertebrate diverge very 

 soon in tlieir develojmient from each other; but two insects, 

 such as a beetle and a honeybee, or any two \('rtebrates, such 

 as a frog and a pigeon, do not diverge from eacli other so soon. 

 That is, all vertebrate iinimals diverge in one (hrection from 

 the other great groups, but all the memluTs of the great group 

 keep together for some time longer. Tlicn the subonhuate 

 groups of the Vertebrata, such as the fishes, th(« 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 rabl)its 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 !)e 

 rougldy 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, com])lex form characteristic of the 

 species to which the animal l)elongs; this development is from 

 simple to comi)lex structural condition; tlie development is 

 the same for all individuals of one s])ecies. While all animals 

 begin development similarly, the coin-se 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 j^assed tlirougii 

 in the developmental course may very much resemble the 



