146 THE POPULAR SCIENCE MONTHLY 



IV. Concluding Remarks 



A summary, properly so called, of the materials of this paper is 

 precluded by the fact that the various sections are in themselves sum- 

 mary reviews of researches carried out upon the most diverse materials. 

 But all these studies have this in common: they are attempts to deter- 

 mine by quantitative methods whether natural selection be a reality, 

 and if so, to measure its intensity. In conclusion, stress may be laid 

 upon two points. 



The first of these is a matter of fact. Evidences of the occurrence 

 of natural selection for many characteristics are rapidly accumulating. 

 That mortality is not random, but differential, and that the intensity of 

 the selective death rate is a problem open to quantitative treatment, 

 are propositions supported by large bodies of sound scientific evidence. 

 Nevertheless, neither the complexity of the phenomena nor the difficul- 

 ties of the collection or of the analysis of the data can be underesti- 

 mated. As yet, only the surface has been touched. The results are all 

 subject to such revision as may be rendered necessary by wider data 

 and narrower analyses. 



The second of these is a question of interpretation. The demon- 

 stration of the existence of a selective death rate in a given case is by 

 no means equivalent to proof that evolutionary change is taking place 

 in the character under consideration. Natural selection may only 

 maintain a characteristic at the stage already attained. Or the force 

 of natural selection may be offset by that of some other factor. Or, 

 again, the variations dealt with may be of a kind not inherited; and 

 without inheritance selection is powerless to effect any change. Indeed, 

 first-hand experience in quantitative work on organic evolution must 

 convince any one that the problem of the methods by which it has taken 

 place is far more recondite than biologists have been wont to consider it. 

 This great complexity demands an attitude of extreme caution in gen- 

 eralization. For the present, we must be content to attempt to measure 

 one possible factor after another in as wide a series of organisms as 

 possible. Having done this, we may hope in time to form a fairly 

 trustworthy conception of the resultant of these forces as they may be 

 combined in nature. 



