Chai-. IV. niODUCTION OF NEW FORMS. 103 



perhaps only at long' intervals ; but in none, as I suspect, can 

 self-fertilization go on for perpetuity. 



Clrcnmstanccs favorable for the Production of Neio Forms 

 through Natural Selection. 



Tliis is an extremely intricate subject. A great amount of 

 variability, under which term individual differences are always 

 included, will evidently be favorable. A large number of in- 

 dividuals, by giving a better chance for the appearance of 

 profitable variations within any given period, will compensate 

 for a lesser amount of variability in each individual, and is, I 

 believe, an extremely important element of success. Though 

 Nature grants long periods of time for the work of natural se- 

 lection, she does not grant an indefinite period ; for, as all or- 

 ganic -beings arc striving to seize on each place in the economy 

 of Nature, if any one species docs not become modified and im- 

 proved in a corresponding degree with its competitors, it will 

 be exterminated. Unless favorable variations be inherited by 

 some at least of the offspring, nothing can be effected by nat- 

 ural sclecticjn. Tlie tendency to reversion may. often check or 

 prevent the work ; but as tliis tendency has not prevented 

 man from forming by selection nimierous domestic races, why 

 .sliould it prevail against natural selection ? 



In the case of methodical selection, a breeder selects for 

 some definite object, and free intercrossing Avill wholly stop 

 his work. But when many men, without intending to alter the 

 breed, have a nearly common standard of perfection, and all 

 try to procure and breed from the best animals, much improve- 

 ment surely but slowly follows from this unconscious process 

 of selection, notwithstanding a large amount of crossing with 

 inferior animals. Thus it Avill be in Nature; for within a con- 

 Ihied area, with some place in its polity not perfectly occu]iicd, 

 natural selection will always tend to preserve all the individ- 

 uals varying in the right direction, though in different degrees, 

 so as better to fill up the imoccupied place. But if the area be 

 very large, its several districts will almost certainly present 

 different conditions of life ; and then, if the same species under- 

 g()(^s modification in different parts, the newly-formed varieties 

 will intercross on the confines of each district. But we shall 

 see in the seventh chapter that intermediate varieties, inhabit- 

 ing an intermediate district, whether the result of the crossing 

 of other varieties, or originally formed with an intermediate 



