THE FACTOES OP ORGANIC EVOLUTION. 9 



mistakes on the part of birds, was increased generation 

 after generation by the more frequent escape of the most- 

 like individuals, until the likeness became thus great. 



But now, recognizing in full this process brought into 

 clear view by Mr. Darwin, and traced out by him with so 

 much care and skill, can we conclude that, taken alone, it 

 accounts for organic evolution ? Has the natural selection 

 of favourable variations been the sole factor ? On critically 

 examining the evidence, we shall find reason to think that 

 it by no means explains all that has to be explained. 

 Omitting for the present any consideration of a factor 

 which may be distinguished as primordial, it may be con 

 tended that the above-named factor alleged by Dr. Erasmus 

 Darwin and by Lamarck, must be recognized as a (co 

 -operator. Utterly inadequate to explain the major part of 

 the facts as is the hypothesis of the inheritance of func 

 tionally-produced modifications, yet there is a minor part 

 of the facts, very extensive though less, which must be I 

 ascribed to this cause. 



When discussing the question more than twenty years 

 ago (Principles of Biology, 166), I instanced the decreased 

 size of the jaws in the civilized races of mankind, as a 

 change not accounted for by the natural selection of 

 favourable variations ; since no one of the decrements by 

 which, in thousands of years, this reduction has been 

 effected, could have given to an individual in which it 

 occurred, such advantage as would cause his survival, 

 either through diminished cost of local nutrition or dimi 

 nished weight to be carried. I did not then exclude, as I 

 might have done, two other imaginable causes. It may 

 be said that there is some organic correlation between 

 increased size of brain and decreased size of jaw : Camper s 

 doctrine of the facial angle being referred to in proof. 

 But this argument may be met by pointing to the many 

 examples of small-jawed people who are also small-brained, 





