150 MAN AND THE LOWER ANIMALS. n 



admitted that the conditions of fertility and steril- 

 ity are very ill understood, and that every day's 

 advance in knowledge leads us to regard the hiatus 

 in his evidence as of less and less importance, when 

 set against the multitude of facts which harmonize 

 with, or receive an explanation from, his doc- 

 trines. 



I adopt Mr. Darwin's hypothesis, therefore, sub- 

 ject to the production of proof that physiological 

 species may be produced by selective breeding; just 

 as a physical philosopher may accept the undu- 

 latory theory of light, subject to the proof of the 

 existence of the hypothetical ether; or as the chem- 

 ist adopts the atomic theory, subject to the proof 

 of the existence of atoms; and for exactly the same 

 reasons, namely, that it has an immense amount 

 of prima facie probability: that it is the only means 

 at present within reach of reducing the chaos of 

 observed facts to order; and lastly, that it is the 

 most powerful instrument of investigation which 

 has been presented to naturalists since the inven- 

 tion of the natural system of classification, and 

 the commencement of the systematic study of 

 embryology. 



But even leaving Mr. Darwin's views aside, the 

 whole analogy of natural operations furnishes so 

 complete and crushing an argument against the 

 intervention of any but what are termed secondary 

 causes, in the production of all the phenomena of 

 the universe; that, in view of the intimate rela- 



