CHAP. xxi. COMPETITION OF RACES. 411 



having the advantage in this respect over most of the herd, 

 as being able to browse on foliage out of their reach, survived 

 them, and transmitted its peculiarity of cervical conformation 

 to its successors. 



By the multiplying of slight modifications in the course 

 of thousands of generations, and by the handing down of 

 the newly-acquired peculiarities by inheritance, a greater and 

 greater divergence from the original standard is supposed to 

 be effected, until what may be called a new species, or, in a 

 greater lapse of time, a new genus, will be the result. 



Every naturalist admits that there is a general tendency in ' 

 animals and plants to vary ; but it is usually taken for granted, 

 though we have no means of proving the assumption to be 

 true, that there are certain limits beyond which each species 

 cannot pass under any circumstances, or in any number of 

 generations. Mr. Darwin and Mr. Wallace say that the 

 opposite hypothesis, which assumes that every species is 

 capable of varying indefinitely from its original type, is not 

 a whit more arbitrary, and has this manifest claim to be pre 

 ferred, that it will account for a multitude of phenomena 

 which the ordinary theory is incapable of explaining. 



We have no right, they say, to assume, should we find that 

 a variable species can no longer be made to vary in a certain 

 direction, that it has reached the utmost limit to which it 

 might, under more favourable conditions, or if more time 

 were allowed, be made to diverge from the parent type. 



Hybridisation is not considered by Mr. Darwin as a cause 

 of new species, but rather as tending to keep variation with 

 in bounds. Varieties which are nearly allied cross readily 

 with each other, and with the parent stock, and such cross 

 ing tends to keep the species true to its type, while forms 

 which are less nearly related, although they may intermarry, 

 produce no mule offspring capable of perpetuating their kind. 



The competition of races and species, observes Mr. Darwin, 



