^^^^ THE NEW EVOLUTION '^^'^'^ 



larval type which is almost always adapted to insure 

 the wide distribution of the species . The unescapable 

 inference is that the balance of the internal organs 

 during this time is not of such a nature as to render 

 possible the existence of such forms as adults. 



Besides this, the fact that successful development 

 from the gastrula to the structural complex character- 

 istic of each major group always follows certain defi- 

 nite lines which, though they may be greatly short- 

 ened, are always undeviatingly the same, would seem 

 to indicate that development outside of or between 

 these lines would result in structural complexes which 

 would be incapable of meeting successfully the con- 

 ditions of existence. Many gastrulas develop in va- 

 rious abnormal ways, but these always die. 



The picture that we get of the developmental 

 history of animals through the study of comparative 

 embryology is that there is not now, and there never 

 has been at any time, any possibility of the existence 

 of economically possible animal types anywhere along 

 the line between a gastrula and the several structural 

 complexes characteristic of the coelenterates and the 

 bilaterally symmetrical major groups. 



The natural conclusion is that the original forma- 

 tion of the original members of these groups resulted 

 from divergencies in early embryonic stages which 

 followed simultaneously every separate line which 

 could lead to an economically possible adult. 



Such lines are limited in number, and for that reason 

 we see the major groups curiously few when we con- 

 sider the enormous multiplicity of minor types within 



