PERFECT ADAPTATION OF ORGANS. 34 1 



stability and variability manifested by every organic form 

 is determined solely by the actual condition of the equi- 

 librium between these two opposite functions. Inheritance 

 is the cause of the stability of species, adaptation the cause 

 of their modification. When therefore some naturalists 

 say that, according to the theory of descent, there ought 

 to be a much greater variety of forms, and others again, 

 that there ought to be a much greater equality of forms, 

 the former under-estimate the value of inheritance and the 

 latter the value of adaptation. The ratio of the interaction 

 between inheritance and adaptation determines the ratio of 

 the stability and variability of organic species at any given 

 period. 



Another objection to the theory of descent, which, in the 

 opinion of many naturalists and philosophers is of great 

 weight, is that it ascribes the origin of organs which act 

 for a definite purpose to causes which are either aimless 

 or mechanical in their operation. This objection seems to 

 be especially important in regard to those organs which 

 appear so excellently adapted for a certain definite purpose 

 that the most ingenious mechanician could not invent a 

 more perfect organ for the purpose. Such are, above all, 

 the higher sense-organs of animals, the eye and ear. If the 

 eyes and auditory apparatus of the higher animals alone 

 were known to us, they would indeed cause great and per- 

 haps insurmountable difficulties. How could we come to 

 the conclusion that the extraordinarily great and wonderful 

 degree of perfection and conformity to purpose which we 

 perceive in the eyes and ears of higher animals, is in every 

 respect attained solely by natural selection ? Fortunately, 

 however, comparative anatomy and the history of develop- 



