48 COSMIC PUILOSOPHY. [pt. ii. 



future inquiries to give us tlie quantitative data requisite for 

 settling this point.^ We cannot yet, indeed, estimate the age 

 of the last gi-eat glacial epoch with any approach to accuracy; 

 yet the age which we assign to this epoch must enter as an 

 important factor into our estimates of the antiquity of pre- 

 ceding epochs. But while this point remains undetermined, 

 it may be noted that even the decision which leaves the 

 smallest time for the operation of unaided natural selection 

 can weaken the Darwinian theory only on the assumption 

 that the agency already alleged by that theory has been the 

 sole factor concerned in forwarding organic evolution; and 

 this assumption, though it may have been made by over- 

 confident disciples of Mr. Darwin, has never been made by 

 Mr. Darwin himself. Mr. Darwin is too profoundly scientific 

 in spirit to imagine that, with all his unrivalled patience and 

 sa'^^acity, he has completely solved one of the most intricate 

 problems with which the student of nature has ever been 

 called upon to deab It is more than likely that future 

 research will disclose other agencies which have cooperated 

 with natural selection in accelerating the diversification of 

 species. Meanwhile the evidence in behalf of the first ten 

 propositions involved in the Darwinian theory is sufficiently 

 strong to make it apparent that a vast amount of specific 

 change must have taken place, and also that natural selection 

 has been a chief factor in producing that change. To the 

 arguments which in our ninth chapter were seen to overthrow 

 the dogma of fixity of species, may now be added the 

 argument that at least one group of clearly-defined agencies 

 is at work, with which, in the long run, the fixity of species 

 must become incompatible. The explanation of the details 

 of specific differentiation may well form the subject of 

 cautious investigation for many generations of observers and 



^ The reader who wishes to see how fallacious all attempts at reachrng th« 

 Age of the eaitli from astronomico-physical arguments are likely to prove with 

 our present resources, may consult Huxley's Lay Sermons, pp. 268 279. 



