TEKDENCT OF SPECIES TO FOEM VABIETIES. 49 



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 vari- 

 ation, 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 teU 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 re- 

 member 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 difference be ever so small, would be slightly 

 favoured, and would tend to live longer, and to survive during 

 that time of the year when food was scarcest ; they would also 

 rear more young, which would tend to inherit these slight pecu- 

 liarities. The less fleet ones would be rigidly destroyed. I can 

 see no more reason to doubt that these causes in a thousand 

 generations would produce a marked effect, and adapt the form of 

 the fox or dog to the catching of hares instead of rabbits, than that 

 greyhounds can be improved by selection and careful breeding. 

 So would it be with plants under similar circumstances. If the 

 number of individuals of a species with plumed seeds could be 

 increased by greater powers of dissemination within its own area 



LINS-. PKQO.— ZOOLOGY. 4 



