374 THE AMEBIC AN NATURALIST [Vol. XLVI 



in that vicinity. Evidence to this effect is seen in the fact that 

 certain shrubs that are aphis-infected for a short time only in the 

 early spring sustain only Adalia Mpunctata. The wild parsnip, 

 which becomes infected with other aphids later, sustains Adalia 

 Mpunctata in numbers smaller than three of its competitors. If 

 we take the herbivorous coccinellids, we will find similarly in 

 some places an abundance of food that they have not touched, 

 while some other patches of suitable plants may be stripped. 



In conclusion, then, we see that in the bryozoa and birds, 

 sustentative selection does not play the dominant role imputed to 

 it. In the lady beetles, where the supply of food is seen to limit 

 their numbers, it is not because there is not food enough, but 

 because the individuals are not properly distributed with refer- 

 ence to that food. The sustentative selection in this case must be 

 differentiated as indirect. The evolutionary significance lies in 

 the fact that where the sustentative selection is indirect, the 

 species may become more abundant through variations which ad- 

 just the individuals better to the food supply. 



I believe there is a fundamental reason for this subordination 

 of sustentative selection. The reproductive rate is not merely 

 an arbitrarily large number, which necessarily causes a severe 

 struggle, but is just such a number as is best adapted, in general, 

 to the needs of the species. The extreme members of that school 

 which emphasizes the importance of the variations at the expense 

 of selection can scarcely object to this, for fecundity is always 

 variable and these differences are known to be inheritable in 



Now the number of progeny which is best adapted to the needs 

 of the species is that number which is large enough to sustain 

 the losses from all non-sustentative causes of death, but not large 

 enough to invoke death by starvation. Such a species is obvi- 

 ously less liable to extermination than one in which the hostile 

 influence of underfeeding always handicaps. If grasshoppers 

 conformed to the Malthusian conceptions of many evolutionists, 

 there should be no alfalfa, for that favorite food would all be 

 eaten up before it could be harvested. The world teems with 

 herbivorous animals of one kind or another, and yet also teems 

 with plants, most of which are eaten by many species of animals. 

 I can see no other explanation than that the species are not ordi- 

 narily subject to sustentative selection, and that when it is, it is 

 generally the indirect selection rather than the direct. 



Indirect sustentative selection is less injurious to a species, be- 



