I 



TWO OBJECTIONS CONSIDERED 



need we wonder that the horse and the ass, with 

 less important differences in general habit, have 

 become partially infertile together, to such an 

 extent that their offspring are hopelessly barren. 

 Though the modus operandi of this change is as 

 yet ill understood, it is nevertheless a change 

 quite in harmony with what we know concern- 

 ing the intimate dependence of the reproductive 

 system upon the rest of the organism. And let 

 us not fail to note that it is the achievement of 

 this change in the capacities of the reproduc- 

 tive system which completes the demarcation 

 between two bifurcating species, and finally pre- 

 vents the indefinite multiplication of intermedi- 

 ate varieties. 



But while this objection has no weight as 

 against the theory of derivation in general, it 

 may fairly be urged that the failure to explain 

 the origination of mutual infertility is, for the 

 present at least, a shortcoming on the part of 

 the theory of natural selection. After the con- 

 clusive arguments brought up in our ninth 

 chapter, the derivation theory will no longer, in 

 the present work, be regarded as on trial : that 

 the higher forms of life are derived from lower 

 forms will be taken as proved. But whether 

 the theory of natural selection has completely 

 fulfilled its proposed task of explaining the mode 

 in which such derivation has been brought 

 about, is quite another question. And while 



VOL. Ill O^ 



