ISOLATION 355 



apply this search to, say, the external characters of a large 

 number of individuals. That is to say, whether we elect to 

 trace the ramifications of evolution by means, say, of the 

 study of the convolution of the intestine of fish or grain-eating 

 birds, or whether we follow out the complexities and peculiarities 

 of the plumage which distinguish some particular group of birds. 

 Very well. What then are the factors which cause these changes ? 

 Are we to ascribe their origin, whether in organs or specific 

 characters, to natural selection, or to some other factor or factors, 

 or to a combination thereof? 



That natural selection is the final arbiter of what shall 

 survive, the sieve through which all the component parts of an 

 organism must pass, may surely be regarded as a well-estab- 

 lished dogma. But it by no means follows that natural selection 

 is a factor in the origin either of species or of specific characters. 



In endeavouring to account for the birth and development 

 of specific characters, we too commonly ignore the fact that 

 living matter is unstable, and can never reproduce itself exactly. 

 Hence the basis of infinite variety. But each new variation may 

 vary in its potentiality. Such as are appreciable to our eyes 

 commonly, it would seem, tend to increase, to intensify in each 

 generation, and, attaining their zenith, finally wane and disappear. 



The evidence afforded by geographical distribution seems 

 to show, pretty conclusively, that " Isolation " is a very power- 

 ful aid to the fostering and development of such potentially 

 progressive variations. 



This being so then, we may take it that natural selection 

 frames her own sumptuary laws but she does not design the 

 fashions. 



Isolation, in whatever form, is, however, like natural selec- 

 tion, incapable of producing variations. These are innate, 

 blastogenic, spontaneous "expression points " of the instability 

 of protoplasm. The more any given group of animals is split 

 up by barriers of whatever kind, the more numerous will, in 

 course of time, become the variation on the common type. 

 That is to say, segregation fosters the survival of variations. 

 Natural selection suffers them, or not. 



Romanes endeavoured to show, indeed, that isolation, under 

 one of two forms — spatial or physiological — was the only 

 possible factor in the origin of species. Though this contention 



