278 THE PRACTICAL APPLICATION OF ECOLOGY 



Table XIII 



Reproduction of Aphis rumicis upon different host varieties. The 

 infestation figures are the mean in each ease of five plants, each plant 

 being infected with a single apterous viviparous female which was 

 allowed to reproduce for a period of fourteen days. 



varieties of apple {vide p. 280). There is evidence, both in the 

 case of varieties of beans and of apple, that those most desirable, 

 from the point of view of human consumption, appear to favour 

 and stimulate the highest reproductive rates in their respective 

 aphid enemies. This, in itself, lends support to the suggestion 

 already made that certain properties, contributing to the greater 

 " resistance " of the wild forms of plants, have become out-bred 

 in the course of repeated selection in cultivation. 



It may be said that, up to the present, comparatively few 

 discoveries of insect-resistant varieties of plants, of proved 

 commercial importance, have come to light. The methods 

 involved in the search for such varieties are either those of selective 

 breeding, varietal hybridisation, or graft hybridisation. Selective 

 breeding entails either the continued selection of individual plants 

 of a crop, observed in the field to betray least injury ; or line 



