ii ON THE TENDENCY OF VARIETIES, ETC. 23 



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 

 assumption 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 Struggle for Existence 



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

 full exertion of all their faculties and all their energies is 

 required to preserve 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 population of a species ; and by a careful consideration of 

 all the circumstances 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 Law of Population of Species 



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 ; and the wild asses of 

 the Tartarian deserts cannot equal in numbers the horses of 



