PAET III 



SUMMARY AND CONCLUSIONS 



THE examples we have been able to give in the 

 first part of this volume make it plain that de- 

 generative evolution exists everywhere. It must be 

 noticed, however, that biological investigation shows 

 that in the evolution of organs certain parts may 

 disappear completely, but also that in the evolution 

 of organisms certain organs may also disappear. 

 This last phenomenon is most common in embryo- 

 logical development, when it is known as ontological 

 abbreviation. 1 Sometimes it is the adult stage that 

 is suppressed, this being possible by what is called 

 psedogenesis a precocious appearance and ripening 

 of the sexual organs. 2 



1 In Scalpellum Stroi'ni, a deep sea Cirripede, the nauplius stage 

 of the larval life is suppressed at least so far as that is a free 

 swimming larva. Here is at least a physiologically complete sup- 

 pression of a whole larval stage. 



2 Axolotl. Most salamanders pass through a larval stage in 

 which they are aquatic and perform their respiration by means of 

 external gills. In this condition they are incapable of reproduc- 

 tion, and must undergo metamorphosis to secure propagation of 

 the species. In the case of Amblystoma, however, a Mexican 

 salamander, the larval form of which is called the Axolotl, repro- 

 duction is possible in the larval stage. Thus most individuals of 



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