Vs 



1il THE NEW EVOLUTION "^^^ 



diately subsequent, stage, and becomes more and more obviously 

 so as the development progresses. Indeed, in some major groups 

 the early gastrulas are characteristic and distinctive. 



Such a condition is capable of but a single interpretation. 

 The only possible conclusion is that there is not now, and there 

 never has been at any time in the past, any linear relationship be- 

 tween the major groups of animals at any stage later than the 

 gastrula. None of them can be assumed to have passed through 

 a stage represented by any of the others in the adult form. 



In all of them there are the same three germ layers — the ecto- 

 derm, representing and arising from the outer layer of cells in the 

 gastrula cup, the endoderm, arising from the layer of cells lining 

 the gastrula cup, and the mesoderm, arising in various ways 

 within the hollow walls of the gastrula cup between the outer 

 ectoderm and the inner endoderm. 



Each of the three germ layers gives rise to definite sets of 

 organs and of structures. Obviously, therefore, a slight modifi- 

 cation of the relationships between them in early embryonic life 

 will produce marked and profound differences in adult animals. 



The relation between the various groups of the more complex 

 animals can only be interpreted on the basis of embryological 

 evidence — and no other evidence is available — as divergence from 

 a common center, this common center being represented by the 

 gastrula. There is no other possible point of common contact. 



We may explain the matter in this fashion. A number of 

 exactly similar hollow hemispheres of modeling clay may be 

 taken and each modeled into a wholly different figure. In this 

 way we could form an infinite series of clay figures which would 

 not show the slightest similarity to each other. But each would 

 be formed of the same clay, and each would have originated as a 

 hollow hemisphere. 



This is about what nature has done in the case of the more com- 

 plex animal forms. Instead of hollow hemispheres of clay, how- 

 ever, nature's units were hollow hemispheres with an outer, an 

 inner, and an intermediate layer of cells, the cells in each of these 

 three groups or layers being capable of forming certain organs or 

 structures only. And besides this, nature's remodeling of the 

 original available materials — the three germ layers — had to 

 produce as a result an animal form with a proper internal chemi- 

 cal and physical balance, and at the same time capable of securing 



[2.48] 



