538 THE POPULAR SCIENCE MONTHLY 



possibly this may ultimately be the fate of the natural-selection theory. 

 In approaching the problem our aim is not to " get positive results," 

 but to find out the truth. Our object is not to bolster up a venerable 

 and out-of -fashion hypothesis, but to test conscientiously that hypothe- 

 sis against concrete data. 



Like other theories, the Darwinian theory must stand or fall accord- 

 ing as the evidence of quantitative biology shall be for or against it. 

 If the micrometer scale and the calculating machine show that any 

 given character has no influence in determining whether an individual 

 shall survive, then for that organ, in that species at the time under 

 consideration, evidence for the potency of selection is wanting. 



The problem is a difficult one ; a priori one would expect most gen- 

 erally to find no changes taking place in the characters of a species 

 because of a selective death rate. If natural selection be actually at 

 work in nature, it is likely that the ancestors of individuals collected 

 in the open will have been subjected to the selective factors which one is 

 trying to measure, and that the race will be held pretty close to the 

 attainable limit of perfection. It is more likely that a selective elimi- 

 nation which recurs every generation will be observed than one of the 

 kind that brings about changes in specific characters. Only in rare 

 cases when a new territory is opened to organisms or some special 

 modification of environment (inorganic or organic) has taken place 

 can we reasonably expect to see the changing of types going forward. 

 Possibly the very difficulties of demonstrating a selective death rate 

 bear witness to its reality ! 



Taking all this for granted, biologists must, it seems to me, face the 

 duty of determining whether natural selection is a fundamental factor 

 in evolution — in short of actually measuring the intensity of the selec- 

 tive death rate. The calipers are ready and their efficiency has been 

 proved. 



The duty to use them is imposed by ideals of good workmanship. 

 " Measure that which is measurable and render measurable that which 

 is not," is the ideal which has hitherto separated the precise from the 

 descriptive sciences. It is the duty as well as the opportunity of the 

 biologist of to-day to break down this distinction. 



The duty to use them is imposed by the history of our science. 

 For nearly half a century natural selection has been one of the chief 

 problems of biology, and it would be cowardly for naturalists of this 

 generation to leave the problem until a definite solution has been 

 secured. 



The duty to use them is imposed by the inability of the biologist to 

 construct for himself a philosophical theory of evolution without nat- 

 ural selection as one of the factors. Yet the philosophical necessity of 

 a given factor does not relieve the scientist from the duty of finding out 

 whether that factor be a reality, and of measuring its intensity. 



