22 ORGANIC EVOLUTION — PHYSICAL 



ancestor reverted to differs more in structure from the 

 last member of the race than a corresponding ancestor 

 in a race that has been more slowly evolved. I must, 

 however, guard myself against possible misconception. 

 When I speak of atavism as a cause of retrogression, I 

 do not necessarily mean that in each case of atavism 

 the whole organism in all its parts retrogresses. This 

 must rarely happen, especially when evolution has been 

 slow. What I do mean is that, generally speaking, 

 atavism, by causing various small retrogressive changes 

 in various parts of the organism, is by its cumulative 

 effects a check to evolution, and even sometimes a 

 cause of retrogression. An organism may undergo 

 retrogressive changes in some of its parts owing to 

 the severity of natural selection becoming relaxed as 

 regards them, at the same time that, owing to increased 

 severity of natural selection, it is undergoing evolution 

 as regards other parts. Thus in the case of a species 

 of bird, the habits of which are becoming less and less 

 aerial, natural selection will cause the legs to undergo 

 evolution, but atavism the wings to undergo continued 

 retrogression backwards, successively from recent an- 

 cestors to remoter ancestors, till, if the power of flight 

 no longer influences the survival rate, there will be an 

 approximation in structure, as far as the wings are con- 

 cerned, to the structure of a remote wingless or rather 

 limbless ancestor; but an approximation only, for, as 

 regards such complex organs as wings, we must not 

 expect to find that a degenerate example in any stage 

 of the retrogression resembles very closely an ancestral 

 wing, in a corresponding stage of the evolution; for 

 instance, we must not suppose that the wing of the 

 apteryx is a close reproduction of a remote ancestral 

 form, for the reason that though the wing in all its 

 parts undergoes retrogression, yet, like the parts of 

 a complex organism, all the parts of the wing do not 



