64 MESSES. C. DABWIN AND A. WALLACE ON THE 



analogy of the domesticated animals, is considered to be higlily 

 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 per- 

 manence 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 

 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 dan- 

 gerous enemies, are the primary conditions which determine the 

 existence both of individuals and of entire species. These con- 

 ditions 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 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 greater or less fecundity of an animal 

 is often considered to be one of the chief causes of its abundance 

 or scarcity ; but a consideration of the facts will show us that it 

 really has little or nothing to do with the matter. Even the least 

 prolific of animals would increase rapidly if unchecked, whereas it 

 is evident that the animal population of the globe must be sta- 

 tionary, or perhaps, through the influence of man, decreasing. 

 Fluctuations there may be ; but permanent increase, except in re- 

 stricted localities, is almost impossible. For example, our own 

 observation must convince us that birds do not go on increasing 

 every year in a geometrical ratio, as they would do, were there not 



