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THE AMERICAN NATURALIST [Vol. XLVI 



THE MALTIIUSIAN PRINCIPLE AND NATURAL 

 SELECTION 



In the last edition of his essay on population, page 2, Malthus 

 laid down the following biological proposition from which he de- 

 rived his well-known sociological conclusion : 



The cause to which I allude is the constant tendency in all animated 

 life to increase heyond the nourishment prepared for it. 



It is incontrovertibly true that there is no bound to the prolific plants 

 and animals, but what is made by their crowding and interfering with 

 each others' means of subsistence. (Italics mine.) 



In plants and irrational animals, the view of the subject is simple. 

 They are all impelled by a powerful instinct to the increase of their 

 species; and this instinct is interrupted by no doubts about providing 

 for their offspring. Wherever, therefore, there is liberty, the power 

 of increase is exerted; and the superabundant effects are repressed 

 afterwards by want of room and nourishment. 



The great influence of this book upon Darwin is well known 

 and so it is not surprising to find him writing in the ' ' Origin of 

 Species," page 60, 



A struggle for existence inevitably follows from the high rate at 

 which all organic beings tend to increase. 



And again on page 72, 



Each organic being is striving to increase in geometrical ratio; each, 

 at some period of its life, during some season of the year, during each 

 generation or at intervals, has to struggle for life and to suffer great 

 destruction. 



It is quite natural, therefore, to find that in the current texts 

 and in the class room, that natural selection is taught as starting 

 from the contrast of a limited subsistence and a very large birth- 

 rate (and I confess that I have been guilty). So strong an im- 

 pression is thus made, that to most people, and unfortunately 

 many sociologists, natural selection has come to mean that factor 

 of evolution which is caused by an excessive birth rate. 



The fact is frequently lost sight of that natural selection effects 

 its results by differential success in mating (sexual selection), 

 and differential fecundity (fecundal selection), as well as by a 

 differential age at death (lethal selection). Even when we con- 

 fine our attention to lethal selection, we shall see that a very 

 large share of its action is in no way dependent upon the ade- 

 quacy of the food supply. Such selection may well be called non- 



