NATURAL SELECTION, ETC. 107 



motives wliich impel the theorist so much farther. 

 Here proofs, in the proper sense of the word, are not 

 to be had. "We are beyond the region of demonstra- 

 tion, and have only probabilities to consider. What 

 are these probabilities? What work will this hy- 

 pothesis do to establish a claim to be adopted in its 

 completeness ? Why should a theory which may 

 plausibly enough account for the diversification of 

 the species of each special type or genus be expanded 

 into a general system for the origination or successive 

 diversification of all species, and all special types or 

 forms, from four or five remote primordial forms, or 

 perhaps from one ? We accept the theory of gravi- 

 tation because it explains all the facts we know, and 

 bears all the tests that we can put it to. We incline 

 to accept the nebular hypothesis, for similar reasons ; 

 not because it is proved thus far it is incapable of 

 j>roof but because it is a natural theoretical deduction 

 from accepted physical laws, is thoroughly congruous 

 with the facts, and because its assumption serves to 

 connect and harmonize these into one probable and 

 consistent whole. Can the derivative hypothesis be 

 maintained and carried out into a system on similar 

 grounds ? If so, however unproved, it would appear 

 to be a tenable hypothesis, which is all that its author 

 ought now to claim. Such hypotheses as, from the 

 conditions of the case, can neither be proved nor dis- 

 proved by direct evidence or experiment, are to be 

 tested only indirectly, and therefore imperfectly, by 

 trying their power to harmonize the known facts, and 

 to account for what is otherwise unaccountable. So 

 the question comes to this : What will an hypothesis 



