IN MODERN SCIENCE. 



51 



existence, saw in this a plausible solution for 

 the question of the progress and the variety 

 of organized beings. 



The original Darwinian theory was soon 

 found to be altogether insufficient to account 

 for the observed facts, because of the tendency 

 of the bare struggle for existence to produce deg- 

 radation rather than elevation ; because of the 

 testimony of geology to the fact that introduction 

 of new species takes place in times of expan- 

 sion rather than of struggle ; because of the 

 manifest tendency of the breeds produced by 

 artificial selection to become infertile and die 

 out in proportion to their deviation from the 

 original types ; and because of the difficulty 

 of preventing such breeds from reverting to 

 the original forms, which seem in all cases to 

 be perfectly equilibrated in their own parts and 

 adapted to external nature, so that varieties 

 tend," as if by gravitative law, to fall back 

 into the original moulds. A great variety of 

 other considerations — as those of sexual selec- 

 tion, reproductive ^acceleration and retardation, 

 periods of more and less rapid evolution, innate 

 tendency to vary at particular times and in par- 

 ticular circumstances — have been imported into 

 the original doctrine. Thus the original Dar- 



