124 DARWINISM chap. 



characters of a species — and so few even among naturalists 

 have ever compared, accurately, considerable numbers of 

 individuals, that the conception of variability as a general 

 characteristic of all dominant and widespread s^oecies, large in 

 its amount and affecting, not a few, but considerable masses of 

 the individuals which make \\p the species, will be to many 

 entirely new. Equally important is the fact that the vari- 

 ability extends to every organ and every structure, external 

 and internal ; while perhaps most important of all is the 

 independent A^xriability of these several parts, each one vary- 

 ing without any constant or even usual dependence on, or 

 correlation Avith, other parts. No doubt there is some such 

 correlation in the differences that exist between species and 

 species — more developed wings usually accompanying smaller 

 feet and vice fersd — but this is, generally, a useful adaptation 

 which has been brought about by natiu'al selection, and does 

 not apply to the individual variability Avhich occurs Avithin 

 the species. 



It is because these facts of variation are so important and 

 so little understood, that they have been discussed in Avhat 

 will seem to some readers wearisome and unnecessary detail. 

 Many naturalists, however, Avill hold that even more evidence 

 is required ; and more, to almost any amount, could easily 

 have been given. The character and A^ariety of that already 

 adduced A\-ill, hoAvever, I trust, convince most readers that 

 the facts are as stated ; Avhile they have been drawn from 

 a sufficiently Avide area to indicate a general principle through- 

 out nature. 



If, noA\', Ave fully realise these facts of variation, along Avith 

 those of rapid multiplication and the struggle for existence, 

 most of the difficulties in the Avay of comprehending hoAv species 

 have originated through natural selection aa-III disapj^ear. For 

 Avhenever, through changes of climate, or of altitude, or of 

 the nature of the soil, or of the area of the country, any 

 species are exposed to new dangers, and have to maintain 

 themselves and proAdde for the safety of their offspring under 

 neAv and more arduous conditions, then, in the A'ariability of 

 all parts, organs, and structures, no less than of habits and 

 intelligence, Ave have the means of producing modifications 

 Avhich Avill certainly bring the species into harmony with its 



