TENDENCY OF SPECIES TO FORM VARIETIES. 9 



power inherent and annually in action in all animals; reflect on the 

 countless seeds scattered by a hundred ingenious contrivances, year after 

 year, over the whole face of the land; and yet we have every reason to 

 suppose that the average percentage of each of the inhabitants of a coun- 

 try usually remains constant. Finally, let it be borne in mind that this 

 average number of individuals (the external conditions remaining the 

 same) in each country is kept up by recurrent struggles against other 

 species or against external nature (as on the borders of the Arctic 

 regions, where the cold checks life), and that ordinarily each individual 

 of every species holds its place, either by its own struggle and capacity 

 of acquiring nourislnnent in some period of its life, from the Qgg up- 

 wards; or by the struggle of its parents (in short-lived organisms, 

 when the main check occurs at longer intervals) with other individuals 

 of the same or different species. 



But let the external conditions of a country alter. If in a small 

 degree, the relative proportions of the inhabitants will in most cases 

 simply be slightly changed; but let the number of inhabitants be 

 small, as on an island, and free access to it from other countries be 

 circumscribed, and let the change of conditions continue progressing 

 (forming new stations), in such a case the original inhabitants must 

 cease to be as perfectly adapted to the changed conditions as they were 

 originally. It has been shown in a former part of this work, that 

 such changes of external conditions would, from their acting on the 

 reproductive system, probably cause the organization of those beings 

 which were most affected to become, as under domestication, plastic. 

 Now, can it be doubted, from the struggle each individual has to obtain 

 subsistence, that any minute variation in structure, habits, or instincts, 

 adapting that individual better to the new conditions, would tell upon 

 its vigour and health? In the struggle it would have a better chance 

 of surviving; and those of its offspring which inherited the variation, 

 be it ever so slight, would also have a better chance. Yearly more are 

 bred than can survive; the smallest grain in the balance, in the long 

 run, must tell on which death shall fall, and which shall survive. Let 

 this work of selection on the one hand, and death on the other, go on for 

 a thousand generations, who will pretend to affirm that it would produce 

 no effect, when we remember what, in a few years, Bakewell effected in 

 cattle, and Western in sheep, by this identical principle of selection? 



To give an imaginary example from changes in progress on an 

 island: — let the organization of a canine animal which preyed chiefly 

 on rabbits, but sometimes on hares, become slightly plastic; let these 

 same changes cause the number of rabbits very slowly to decrease, 

 and the number of hares to increase ; the effect of this would be that the 

 fox or dog would be driven to try to catch more hares ; his organization, 

 however, being slightly plastic, those individuals with the lightest forms. 



