PERFECT ADAPTATION OF ORGANS. 341 
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 wnteraction 
between inheritance and adaptation determines the ratio of 
the stability and variability of organie 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- 
