388 EVOLUTION AND GENETICS 



of evolution by Moritz Wagner, with some reason, but in consider- 

 ing Wagner's work Darwin looked upon isolation as of minor 

 importance. He recognized, however, that it must necessarily 

 be a contributing factor in natural selection in some cases. While 

 it may perpetuate and even emphasize indifferent characters, it 

 may also subject the organism to specific conditions of environ- 

 ment or organic association which some variations may meet more 

 readily than others, and to this extent it is an effective stimulus 

 to the selection of the useful characters. 



Overproduction and Crowding. Organic relationships, based 

 on the principle of overproduction and crowding suggested by 

 Malthus' work, were chiefly emphasized by Darwin as the imme- 

 diate cause of natural selection. Such relationships may, as we 

 have already seen, be due either to the competition of individuals 

 of the same species or those of different species, and may bring 

 about very different responses. 



The fact of overproduction cannot be doubted. If the eggs 

 produced by single animals, or the seeds of single plants, were 

 all to mature, in a few generations one species would crowd the 

 earth to its own extinction. Darwin wrote: "Even slow-breed- 

 ing man has doubled in twenty-five years, and at this rate, 

 in less than a thousand years, there would literally not be standing 

 room for his progeny. Linnaeus has calculated that if an annual 

 plant produced only two seeds — and there is no plant so unpro- 

 ductive as this — and their seedlings next year produced two, and 

 so on, then in twenty years there would be a million plants." 

 But we have already considered cases of rapid increase of animals 

 of economic importance when freed from natural checks. When 

 we consider that oysters commonly produce 16,000,000 eggs, and 

 some fishes even more, we realize the vastness of overproduction 

 and the possible effects of interruption of the natural balance in 

 such species. Lull states the striking facts that if all the progeny 

 of one oyster survived and multiplied and so on until there were 

 great-great-grandchildren, these would number 66,000,000,000,- 

 000,000,000,000,000,000,000,000, and the heap of shells would be 

 eight times the size of the earth! 



Competition. Such increase can only result in competition of 

 various kinds. Animals must have other organisms for food, and 

 so much of the surplus is destroyed. Plants must have sunlight 

 and moisture, and so they cannot be crowded beyond the available 



