420 THE POPULAR SCIENCE MONTHLY. 



exceedingly difficult thing to distinguish, in the performance of a com- 

 plex machine, the part played by certain forces, as gravity and inertia, 

 everywhere operating according to known laws. Nevertheless, no one 

 would doubt that gravity and inertia do cooperate in the machine, nor 

 should we for a moment hesitate to ascribe to the one or the other of 

 these two forces whatever subordinate eflPects are only explainable by 

 each respectively. And so in the present case. Natural selection is 

 not, like the supposititious laws of organic structure, an empirical rule 

 which may to-morrow, perhaps, prove nugatory. Neither, indeed, is it, 

 like mathematico-physical laws, an infallible leading principle govern- 

 ing material events. But, as being a proposition deduced by a chain 

 of valid inferences from universally admitted facts, and at the same time 

 a proposition necessary in se, natural selection stands midway between 

 a rule and a law, though it comes nearer to the latter. Hence, of the 

 two evolution principles of organic nature, laws of structure and natural 

 selection, the latter is in theory the surer, whatever may be its short- 

 comings in practice. 



Undoubtedly it were much to be desired that we could in the in- 

 dividual instance demonstrate the woi'king of natural selection, and 

 follow the process step by step. But this we can not reasonably expect 

 to do. Between the work of natural selection for one generation and 

 the result after 100,000 generations, there subsists about the same 

 ratio as between diflFerential and integral. How seldom it is that we 

 are able to understand this latter ratio, even though we subject it to 

 calculation ! But do we for that reason question the correctness of our 

 integration ? The corresponding problem, in the present instance, 

 would be to investigate and ascertain the evolution of a species through 

 an endless line of generations and under diverse external conditions, 

 while at the same time, as has been already stated, unintelligible laws 

 of structure, working either not adaptively or only accidentally so, enter 

 the problem as undetermined constants, or even as undetermined func- 

 tions. Though this can not be done, it does not follow that we must 

 misapprehend the ratio between the differential and the integral found 

 for us by nature, as though by a calculating-machine. 



Thus, then, so far as the validity of the principle in general is con- 

 cerned, it may be for us a matter of indifference whether or not in the 

 individual instance we can discern and demonstrate the operation of 

 natural selection. As things stand, it must be operative, and the only 

 question is, whether its influence, as regards quantity, is comparable 

 to that of the laws of structure, or whether other more powerful influ- 

 ences obliterate its effects, so that the adaptation prevailing throughout 

 nature would be attributable solely to the action of these laws. In 

 view of this question, the following appears to me to be the proper 

 attitude of the investigator of nature : 



That natural selection can perform what we must ascribe to it in 



