EVOLUTION AND DEGENERATION OF ORGANS 23 



the original formation of these organs in their 

 ancestral forms, their development in the indivi- 

 dual, the phylogenetic evolution of function, and the 

 great variation resulting from individual adaptation. 

 Before entering upon our immediate subject, it 

 would be as well to define its limits by saying 

 a few words concerning the questions not under 

 immediate consideration. 



1. Original formation of organs in ancestral 

 forms. Little is known concerning the primary 

 origin of organs, and their development before their 

 assumption of the aspect and function by which 

 we know them. What were the rudimentary 

 leaves like in the ancestors of flowering plants? 

 What were the eyes of the first vertebrates like ? 

 Did these organs develop from existing organs ful- 

 filling other functions, or were they formed inde- 

 pendently ? However that may be, if they arose 

 from other organs, we know nothing of the modifi- 

 cations which they underwent ; and if they were 

 formed independently, we need not discuss the 

 fact here, for in that case the organ would not 

 have developed by transformation. Having once 

 been formed, it would develop and improve, and 

 this process would not necessarily have been 

 attended by partial degeneration. 



2. Development of organs in the individual. In 

 the course of embryological development, organs 

 do not exactly repeat the successive phases 



