to form Varieties. 



6295 



recurrent struggles against other species or against external nature 

 (as on the borders of the Arctic regions, where the cold checks lite), 

 and that ordinarily each mdividual of every species holds its place, 

 either by its own struggle and capacity of acquiring nourishment in 

 some period of its life, from the egg upwards ; or by the struggle of 

 its parents (in short-lived organisms, when the main check occurs at 

 longer intervals) with other individuals of the saine 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 condi- 

 tions, 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 gene- 

 rations, 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, longest limbs, and best eyesight, let the diflerence 

 be ever so small, would be slightly favoured, and would tend to live 



