UNSOLVED PROBLEMS OF SCIENCE. 45 



The gravest objection to the doctrine of natural selection was 

 expressed by Weismann in a paper published a few months ago, 

 not as agreeing to the objection, but as resisting it ; and therefore 

 his language may be taken as an impartial statement of the diffi- 

 culty. " We accept natural selection," he says, " not because we 

 are able to demonstrate the process in detail, not even because we 

 can with more or less ease imagine it, but simply because we must 

 because it is the only possible explanation that we can conceive. 

 We must assume natural selection to be the principle of the ex- 

 planation of the metamorphoses, because all other ajiparent prin- 

 ciples of explanation fail us, and it is inconceivable that there 

 could yet be another capable of explaining the adaptation of 

 organisms without assuming the help of a principle of design," 



There is the difficulty. We can not demonstrate the process 

 of natural selection in detail ; we can not even, with more or less 

 ease, imagine it. It is purely hypothetical. No man, so far as 

 we know, has ever seen it at work. An accidental variation may 

 have been perpetuated by inheritance, and in the struggle for ex- 

 istence the bearer of it may have replaced, by virtue of the sur- 

 vival of the fittest, his less improved competitors ; but, as far as 

 we know, no man or succession of men have ever observed the 

 whole process in any single case, and certainly no man has re- 

 corded the observation. Variation by artificial selection, of course, 

 we know very well ; but the intervention of the cattle breeder 

 and the pigeon fancier is the essence of artificial selection. It is 

 effected by their action in crossing, by their skill in bringing the 

 right mates together to produce the progeniture they want. But 

 in natural selection who is to supply the breeder's place ? Unless 

 the crossing is properly arranged, the new breed will never come 

 into being. What is to secure that the two individuals of oppo- 

 site sexes in the primeval forest, who had been both accidentally 

 blessed with the same advantageous variation, shall meet, and 

 transmit by inheritance that variation to their successors ? Un- 

 less this step is made good, the modification will never get a start ; 

 and yet there is nothing to insure that step, except pure chance. 

 The law of chances takes the place of the cattle breeder and the 

 pigeon fancier. The biologists do well to ask for an immeasura- 

 ble expanse of timQ. if the occasional meetings of advantageously 

 varied couples from age to age are to provide the pedigree of modi- 

 fications which unite us to our ancestor the jellyfish. Of course 

 the struggle for existence, and the survival of the fittest, would 

 in the long run secure the predominance of the stronger breed 

 over the weaker. But it would be of no use in setting the im- 

 proved breed going. There would not be time. No possible vari- 

 ation which is known to our experience, in the short time that 

 elapses in a single life between the moment of maturity and the 



