en. XL] TWO OBJECTIONS CONSIDERED. 45 



that tlie 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 nevei'theless a change quite in harmony 

 with what we know concerning the intimate dependence of 

 the reproductive system upon the rest of the organism. And 

 let lis not fail to note that it is the achievement of this 

 change in the capacities of the reproductive system which 

 completes the demarcation between two bifurcating species, 

 and finally prevents the indefinite multiplication of inter- 

 mediate 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 conclusive arguments 

 brought up in our ninth chapter, the derivation theory will 

 no loi7ger, 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 ex- 

 plaining the mode in which such derivation has been brought 

 about, is quite another question. And while admitting the 

 full force of the considerations alleged by Mr. Darwin, in his 

 admirable chapter on Hybridism, it seems to me that there is 

 a gap at this point which further research will be required 

 to fill.^ As Prof. Huxley reminds us, " it must not be for- 



' I donht if the hypothesis of natural selection, taken alone, will afFord the 

 Bolutiou of this problem. It seems more likely that such considerations will 

 have to enter as are presented in Mr. Spencer's Principles of Biology, vol. L 

 ]ip. 209-201. Concerning what may be called the " dynamics of heredity," 

 we kiiow as yet but little ; but as far as speculation has already gone, Mr. 

 Darwin's theory of pangenesis seems to me decidedly inferior to Mr. Spencer's 

 theory of physiological units. I do not discuss these theories here, because 

 it is not necessary for the general purposes of this work It may do no 

 aamu however, to remind some oi° my readers that " pangenesis " is merely 



