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THE AMERICAN NATURALIST [Vol. XL VI 



is misleading, and a great mass of biological and sociological 

 writing has been led into error by it. Of course, there can be no 

 objection to the position that since the number of progeny ex- 

 ceeds the number of parents, there must be many premature 

 deaths, or the species will increase in numbers. But this is a 

 truism. The real essence of Malthusianism lies in the conclusion 

 that a sustentative struggle must arise, and there lies the error. 

 The Malthusian conception of the rate of reproduction is that the 

 rate is such that the food supply must be overtaxed and a struggle 

 for existence will take place. The conception here urged is that 

 each species has such a reproduction rate as will suffice to with- 

 stand the premature deaths and sterility of some individuals, and 

 yet not so large as to press normally upon the limits of the food 

 supply. 



I believe the common over-estimation of the role of sustenta- 

 tive selection, and the neglect of the non-sustentative, is largely 

 historical in origin, and that it is maintained by improper teach- 

 ing. 



In teaching natural selection, the fault is generally made of 

 starting with the Malthusian contrast between the limitation of 

 the food supply and the rate of reproduction. The current con- 

 ception will not be righted until we learn to teach natural selec- 

 tion more correctly. While the rate of reproduction is the 

 proper place from which to start, this should be treated, not as a 

 fixed quantity to which nature must accommodate itself, but as 

 that number which just exceeds the great number of premature 

 deaths and suffices to replace the parents. Then the premature 

 death of the 999 must be explained. After an examination of 

 these causes the student can not but grasp the master role of the 

 non-sustentative form. 



Finally, then, we see from these considerations, that the com- 

 mon assumption that every species is as common as it can be, 

 because it is living up to the limits of subsistence is erroneous. 

 A relaxation of any of the causes of death in most cases will in- 

 crease the numbers. 



for Experimental Evolution at Cold Spring Harbor, N. Y., of the Carnegie 

 Institution of Washington.] 



Roswell H. Johnson. 



