522 TEE POPULAR SCIENCE MONTHLY 



the inadequacy of space and food for all, certain individuals are by 

 virtue of their characteristics better fitted to survive under their ex- 

 isting conditions of life than others. 



For Darwin these were merely hypotheses to be conscientiously 

 tested against all known facts. With the facts which Darwin pains- 

 takingly collected and sifted they seemed to agree so well that natural- 

 ists accepted his theory as the best explanation of the diversity of the 

 organic world. Fifty years has furnished, it seems to me, a highly 

 satisfactory substantiation of the first two propositions. We have for- 

 gotten that it was once necessary to convince biologists that variations 

 do occur, and are now trying to measure the frequency and amount of 

 variations, to determine what their proximate causes are, and to classify 

 them. Variation and heredity are so intimately linked together that 

 one can not be extensively investigated without considering the other. 

 Since the pioneer work of Galton a few men have been actively engaged 

 in the measurement of the intensity of heredity, and of recent years 

 many more have been occupied with the experimental study of the 

 physiological phases commonly known as genetics. 



In consequence of this activity our knowledge of variation and in- 

 heritance is much more extensive than was that of Darwin — it would be 

 to our shame if this were not true — but if biologists could all escape 

 for a moment from the limitations of vision imposed by the tangle of 

 post-Darwinian detail and by assumption and subsidiary qualification, 

 and could look at the problems and the data of organic evolution as a 

 whole and in the large I think they would be almost unanimous in re- 

 garding these two first propositions as so well established that they 

 present no difficulty to the acceptance of the Darwinian theory. 



The insecurity of the Darwinian tripod is to be seen in the weak- 

 ness of the proposition that natural selection moulds the species by 

 eliminating variations not adapted to the environment. While the first 

 two hypotheses have been replaced by the masonry of quantitative sci- 

 ence, the third remains largely a hypothesis, weakly reinforced by anal- 

 ogy and by the indirect evidence of adaptation. 



To make more widely known the fact that natural selection is 

 capable of quantitative treatment, of direct measurement, just as are 

 variation and inheritance, is the purpose of this essay. It is not a 

 brief for Darwinism, but a plea for direct quantitative researches into 

 one of the more neglected problems of organic evolution. 



II. The Problems of Selection 

 By the word selection in its most general evolutionary sense we 

 mean merely that those individuals which leave offspring are not on 

 the average representative of their generation, but that they differ in 

 some regards from those which do not survive to be parents. In sta- 

 tistical terminology they are not a random sample of the population. 



