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THE AMERICAN NATURALIST [Vol, L 



house through the accretions of successive generations; 

 or is like some medieval cathedral to which each genera- 

 tion has added some sculptures, some stones to the 

 steeple, some new stained-glass windows. Organisms, 

 the theory runs, began simple,— they varied, why or how 

 no matter; variation is given and is in all directions. 

 Positive variations toward greater complexity were 

 usually advantageous and individuals showing such 

 elbowed out of existence their less-favored cousins and 

 established a new and higher level from which evolution 

 might proceed. 



This view has certain resemblances to an old view of 

 ontogeny. The embryo is simple. Food is added and 

 food makes this part and that grow. Why only certain 

 parts grow in the presence of food and not all— the dif- 

 ferential nature of growth— that is given. It is the 

 nature of the organism that all parts should not grow 

 equally; but the essential thing is that food and water 

 and heat are the things that add parts and make the 

 embryo larger and more complex. 



This view of development has, I think it will be ad- 

 mitted, now become generally abandoned. To-day we 

 recognize rather that the egg or embryo is not so simple, 

 but that, on the contrary, it has wrapped up in it all of 

 the potentialities that are eventually realized ; potentiali- 

 ties that will be realized, however, only if conditions of life 

 (food, water, heat, etc.) are appropriate. 



The alternative view of evolution, like the modern view 

 of embryonic development, lays more stress on the in- 

 ternal factors of evolution. It postulates that the primi- 

 tive organisms, like the eggs, are not so simple as they 

 look but have a molecular constitution of great com- 

 plexity ; and, just as the egg has a mechanism by virtue 

 of which (under favorable conditions) it develops, so the 

 ancestral protoplasm had a mechanism by virtue of which 

 (under favorable conditions) it evolved. We do not 

 know much about the specific molecular machinery that 

 determines the specific nature of differential growth, but 



