ORGANIC EVOLUTION" — PHYSICAL 29 



traits are essential to survival, and therefore evolution 

 is more rapid as regards them. Many traits essential 

 in a Ax-ild state for survival, in a cultivated or domesti- 

 cated state are not essential, and therefore as regards 

 them there is retrocession. In a few generations 

 " sports " appear ; but these sports, if I am right, must 

 generally he examples of retrogression, not of evolution 

 (retrogression in traits which were essential to the wild 

 ancestors, but are no longer essential to the cultivated or 

 domesticated descendants, or far-reaching retrogression 

 in traits rapidly acquired under artificial selection) ; 

 must generally be due to revei-sion to the ancestral 

 form, not to an advance beyond it. 



Again, atavism is not the only cause of retrogression. 

 Evolution may be a cause of apparent retrogression, as 

 in the case of certain insects, which, living as they do 

 in storm-swept islands, are exposed when flying to the 

 danger of being carried to sea. In them natural 

 selection, reversed selection as it is called in such 

 cases, has co-operated with atavism to deprive them of 

 the power of flight. Therefore, if this theory of retro- 

 gression be correct, it may afford us a not unimportant 

 insight into the past life-history of species, and enable 

 us to decide what retrogressive changes are due to 

 atavism and what to natural selection. For instance, 

 man as we know has descended from a hairy ancestor : 

 if his present partially hairless condition is due to atavism, 

 then since he has reverted in this respect to a remote 

 hairless ancestry, his embryo Avill not be hair)-, nor will 

 he in cases of atavism be hairy. We know that this 

 is not the case ; therefore his present hairlessness is due 

 mainly to natural selection (reversed selection), possibly 

 to that form of natural selection known as sexual 

 selection. 



This theory has not, so fai' as I am aware, been 

 propounded before, and it is opposed to another theory 



