THE EVOLUTIONARY CONCEPT 



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animals and cultivated plants and second in the concept (derived from 

 Malthus) of corn-petition for success in an overpopulated world. His reason- 

 ing was as follows: 



1. The Effects of Artificial Selection. From a few original kinds of useful 

 animals and plants, men have succeeded in producing a great variety of cultural 

 races by breeding from those individuals that possessed the most desirable charac- 



Fig. 23.2. Some leaf-mimicking tropical insects. Darwin sought an explanation of such 

 resemblance in the theory of natural selection. Above, green leaflike katydids (Orthoptera). 

 Lower left, a green leaflike mantid (Orthoptera). Lower right, four species of butterflies 

 that resemble dead leaves. (Courtesy Ward's Natural Science Establishment, Inc.) 



teristics. As Darwin was aware, many of the improved varieties have appeared 

 at a single leap as "sports." Others (and Darwin believed these to be the more 

 numerous) were obtained by breeding from those individuals of a variable stock 

 that showed the beginnings of desired modifications, by crossing of strains, and 

 by repetition of the selection through subsequent generations. Darwin transferred 

 the idea of selective breeding to events as they occur without human intervention 

 and sought for selective agencies at work on species in the wild state. 



