14 THE VARIATION OF ANIMALS IN NATURE 



depends not only on a morphological criterion but also 

 on differences in ability to interbreed, populations being 

 more or less isolated from one another. We may distinguish 

 between populations separated by temporary topographical 

 barriers, populations which, if the barriers were removed, 

 would interbreed freely and soon become homogeneous, and 

 those separated by permanent reproductive barriers either of 

 instinct or due to sterility. In the latter case the populations 

 remain distinct even when inhabiting the same area. The 

 study of variation, therefore, is much concerned with the 

 origin and causes of isolation. 



(iv) In different individuals of a population are many 

 more or less peculiar characters, but only those will be called 

 specific which are found in association in the bulk of the 

 members. Thus the specific characters are more or less 

 correlated with one another and we have to investigate the 

 origin and causes of this correlation. 



(v) The divergence of populations depends not only on 

 the occurrence of new variations but on their accumulation 

 to give rise to those groups of characters by which species are 

 recognised. Any new character to become specific, if it does 

 not first appear in a number of individuals simultaneously, 

 must arise in one or a few individuals and then spread through 

 the species. We must further consider, then, the spread of 

 new characters within the population. 



(vi) We have finally to consider what is the relationship 

 between the establishment of groups and the main tendencies 

 of evolution. It is almost universally held that the main 

 adaptive divergences which constitute the most striking feature 

 of evolution are merely group-divergences progressively raised 

 to a higher power by the continued operation of the same 

 processes that produced group-formation. We consider that 

 this is a questionable doctrine. Chapters IX and X are 

 devoted to a consideration of the relation between variation 

 and organisation. 



The problems enumerated above will be treated in the 

 following order. In Chapter II we consider the origin of 

 variation. In Chapter III we enumerate the types of 

 groups recognised as the result of various methods of study 

 (systematic, genetical, etc.), and in Chapter IV we detail how 

 variants and groups of variants are actually found in nature. 



