5 1 4 Studies in the Theory of Descent. 



arrived at if, instead of fixing our attention upon 

 the different stages of a systematic group in their 

 phyletic development, we only compare the diffe- 

 rent functional parts (organs in the wide sense) of 

 the organisms. 



A complete parallel can be drawn between the 

 two classes of developmental phenomena. From 

 the very different systematic values attached by 

 taxonomists to this or that organ in a group of 

 animals, it may be concluded that the individual 

 parts of an organism are to a certain extent inde- 

 pendent, and that each can vary independently, 

 when affected either entirely alone or in a prepon- 

 derating degree by transforming impulses, without 

 all the other parts of the organism likewise suffer- 

 ing transformation, or at least without their be- 

 coming modified in an equal degree. Did all the 

 parts and organs in two groups of animals diverge 

 from each other to the same extent, the systematic 

 value of such parts would be perfectly equal ; we 

 should, for example, be able to distinguish and 

 characterize two genera of the family of mice by 

 their kidneys, their liver, their salivary glands, or 

 by the histological structure of their hair or 

 muscles, or even by differences in their myology, 

 &c. equally as well as by their teeth, length of 

 toes, &c. It is true that such a diagnosis has yet 

 to be attempted ; but it may safely be predicted 

 that it would not succeed. Judging from all the 

 facts at present before us, the individual parts and 



