148 MAN AND THE LOWER ANIMALS. n 



It cannot be doubted, I think, that Mr. Dar- 

 win has satisfactorily proved that what he terms 

 selection, or selective modification, must occur, 

 and does occur, in nature; and he has also proved 

 to superfluity that such selection is competent to 

 produce forms as distinct, structurally, as some 

 genera even are. If the animated world presented 

 us with none but structural differences, I should 

 have no hesitation in saying that Mr. Darwin had 

 demonstrated the existence of a true physical cause, 

 amply competent to account for the origin of liv- 

 ing species, and of man among the rest. 



But, in addition to their structural distinctions, 

 the species of animals and plants, or at least 

 a great number of them, exhibit physiological 

 characters — what are known as distinct species, 

 structurally, being for the most part either alto- 

 gether incompetent to breed one with another; or 

 if they breed, the resulting mule, or hybrid, is 

 unable to perpetuate its race with another hybrid 

 of the, same kind. 



A true physical cause is, however, admitted to 

 be such only on one condition — that it shall 

 account for all the phenomena which come within 

 the range of its operation. If it is inconsistent 

 with any one phenomenon, it must be rejected; 

 if it fails to explain any one phenomenon, it is so 

 far weak, so far to be suspected; though it may 

 have a perfect right to claim provisional accept- 

 ance. 



