MEASUREMENT OF NATURAL SELECTION 523 



For the evolutionist a question of fundamental importance is this : 

 Do the offspring of selected individuals differ from unselected individ- 

 uals of the same population? This is, however, really a problem of 

 variation and heredity and not of selection at all. Given variations 

 which are heritable, stringent selection will change the type of the 

 population. If after this change of type no more heritable variations 

 occur, selection can effect no further change. 



The history of cultivated varieties shows us that much can be ac- 

 complished by selection, but neither the history of animals and plants 

 under domestication nor any amount of experimental evidence would be 

 sufficient to demonstrate the correctness of the third Darwinian propo- 

 sition. 



This has often been recognized. " The real difficulty of Darwin's 

 theory is the transition from artificial to natural selection," said Paul 

 Janet. Darwin himself frankly tells us, " I soon perceived that selec- 

 tion was the keystone of man's success in making useful races of ani- 

 mals and plants. But how selection could be applied to organisms liv- 

 ing in a state of nature remained for some time a mystery to me." 



Experimental breeding and statistical studies in variation and 

 heredity can teach us much, but evolution, for the most part, has oc- 

 curred outside the breeding pen. Some species have originated in the 

 greenhouse and some in a hanging drop culture, but most species have 

 come into existence and biological dynasties risen to dominance and 

 sunk into decadence in the fields and swamps and mountains where or- 

 ganisms live in competition and cooperation, as host and parasite, as 

 destroyer and destroyed. From the standpoint of evolution the vital 

 question concerning selection is: Does selection (natural, sexual or 

 genetic) occur in nature? 



Is a selective death rate such an important factor that equipped with 

 proper instruments the biologist can go out into free nature and meas- 

 ure its intensity? If he can, then the Darwinian theory of evolution 

 must detain us longer; if he can not, we must lay Darwinism on one 

 side, and maintain towards it, as towards all other theories for which 

 critical evidence is wanting, an attitude of agnosticism. 



III. The Measurement of Selective Elimination' 

 The hypothesis of the existence of the evolutionary factor known as 

 natural selection is dependent upon the assumption that individuals 

 vary in their capacity to withstand the pressure of their environment, 

 and that the differences in resistance to untoward external conditions 

 are associated with and due to differences in the physical, physiological 

 or psychical characteristics of the organism. 



It does not assume that every death is selective. Many are due to 

 factors which eliminate irrespective of any particular character; many 

 survivals are due to a fortuitous combination of favorable environmental 



