1 68 THE HISTORY OF CEEATION. 



viduals will propagate themselves by the side of the former. 

 Moreover, man can prevent the crossing of the original and 

 the new form, which in natural selection is often unavoidable. 

 If such a crossing, that is, a sexual connection, of the new 

 variety with the original forms takes place, the offspring 

 thereby produced generally returns to the original character. 

 In natural selection, such a crossing can be avoided only 

 when the new variety by migration separates from the origi- 

 nal and isolates itself 



Natural selection therefore acts much more slowly; it 

 requires much longer periods than the artificial process of 

 selection. But it is an essential consequence of this difier- 

 ence, that the product of artificial selection disappears much 

 more easily, and that the new form returns rapidly to the 

 earlier one, which is not the case in natural selection. The 

 new species arising from natural selection maintain them- 

 selves much more permanently, and return much less easily 

 to the original form, than is the case with products of artifi- 

 cial selection, and accordingly maintain themselves during a 

 much longer time than the artificial races produced by man. 

 But these are only subordinate differences, which are ex- 

 plained by the different conditions of natural and artificial 

 selection, and in reality are connected only with differences 

 in the duration of time. The nature of the transformation 

 and the means by which it is produced are entirely the 

 same in both artificial and natural selection. (Gen. Morph. 

 ii. 248). 



The thoughtless and narrow-minded opponents of Darwin 

 are never tired of asserting that his theory of selection is 

 a groundless conjecture, or at least an hypothesis which has 

 3^et to be proved. That this assertion is completely un- 



