6300 



Tendency of Species 



belief in what are called "pernianent or true varieties," — races of 

 animals which continually propagate their like, but which differ so 

 slightly (although constantly} from some other race, that the one is 

 considered to be a variety of the other. Which is the variety and 

 which the original species^ there is generally no means of determining, 

 except in those rare cases in which the one race has been known to 

 produce an offspring unlike itself and resembling the other. This, 

 however, would seem quite incompatible with the " permanent in- 

 variability of species," but the difficulty is overcome by assuming that 

 such varieties have strict limits, and can never again vary further from 

 the original type, although they may return to it, which, from the 

 analogy of the domesticated animals, is considered to be highly 

 probable, if not certainly proved. 



It will be observed that this argument rests entirely on the assump- 

 tion, that varieties occurring in a state of nature are in all respects 

 analogous to or even identical with those of domestic animals, and 

 are governed by the same laws as regards their permanence or further 

 variation. But it is the object of the present paper to show that this 

 assumption is altogether false, that there is a general principle in 

 nature which will cause many varieties to survive the parent species, 

 and to give rise to successive variations departing further and further 

 from the original type, and which also produces, in domesticated 

 animals, the tendency of varieties to return to the parent form. 



The life of wild animals is a struggle for existence. The full 

 exertion of all their faculties and all their energies is required to pre- 

 serve their own existence and provide for that of their infant offspring. 

 The possibility of procuring food during the least favourable seasons, 

 and of escaping the attacks of their most dangerous enemies, are the 

 primary conditions which determine the existence both of individuals 

 and of entire species. These conditions will also determine the popu- 

 lation of a species ; and by a careful consideration of all the circum- 

 stances we may be enabled to comprehend, and in some degree to 

 explain, what at first sight appears so inexplicable— the excessive 

 abundance of some species, while others closely allied to them are 

 very rare. 



The general proportion that must obtain between certain groups of 

 animals is readily seen. Large animals cannot be so abundant as 

 small ones ; the Carnivora must be less numerous than the Herbivora ; 

 eagles and lions can never be so plentiful as pigeons and antelopes ; 

 the wild asses of the Tartarian deserts cannot equal in numbers the 

 horses of the more luxuriant prairies and pampas of America. The 



