THE ORIGIN OF SPECIES. 51 



very same blood tend to be sterile, and somewhat re- 

 moter unions diminish this tendency, and when they 

 have diverged into two varieties the cross-breeds be- 

 tween the two are more fertile than either pure stock 

 yet when they have diverged only one degree more 

 the whole tendency is reversed, and the mongrel is ster- 

 ile, either absolutely or relatively. He who explains 

 the genesis of species through purely natural agencies 

 should assign a natural cause for this remarkable result ; 

 and this Mr. Darwin has not done. "Whether original or 

 derived, however, this arrangement to keep apart those 

 forms which have, or have acquired (as the case may 

 be), a certain moderate amount of difference, looks to 

 us as much designed for the purpose, as does a ratchet 

 to prevent reverse motion in a wheel. If species have 

 originated by divergence, this keeps them apart. 



Here let us suggest a possibly attainable test of the 

 theory of derivation, a kind of instance which Mr. 

 Darwin may be fairly asked to produce viz., an in- 

 stance of two varieties, or what may be assumed as 

 such, which have diverged enough to reverse the move- 

 ment, to bring out some sterility in the crosses. The 

 best marked human races might offer the most likely 

 case. If mulattoes are sterile or tend to sterility, as 

 some naturalists confidently assert, they afford Mr. 

 Darwin a case in point. If, as others think, no such 

 tendency is made out, the required evidence is want- 

 ing. 



A fourth and the most formidable difficulty is that 

 of the production and specialization of organs. 



It is well said that all organic beings have been 

 formed on two great laws : unity of type, and adap- 



