298 THE PRACTICAL APPLICATION OF ECOLOGY 



of the cases studied. The phenomenon is most in evidence 

 where the species concerned are able to exercise freedom of 

 choice in the selection of their food-plants. In the absence of 

 such freedom, under experimental conditions, it has already been 

 shown that host-preference may be insufficiently habituated to 

 preclude an insect from adopting new hosts. In cases where 

 host-preferences are most strongly developed they appear to have 

 led to the evolution of definite biological, or phytophagic, races 

 which are now known to occur in widely separated groups of 

 insects. Such races may further exhibit differences of form, 

 colour and behaviour. 



The factor or factors involved in the exercise of host-selection 

 are, as yet, very little understood, and various hypotheses have 

 been advanced to account for the repetition of similar habits 

 in this respect from generation to generation. There is 

 considerable evidence in favour of the contention that for many 

 insects polyphagy is phylogenetically the older habit, oligophagy 

 and monophagy being more recent developments. Both Hering 

 for Lepidoptera and Mordwilko for Aphididas, who have specially 

 considered this problem, hold that it applies to the groups referred 

 to. A very large number of records occur in recent entomological 

 literature of insects resorting at times to unusual, or hitherto 

 unknown, food-plants. It may be argued that, in certain cases, 

 this sporadic habit is to be interpreted as a physiological mutation 

 in the species concerned. On the other hand, it appears equally 

 probable that such instances may be examples of the exercise of 

 a latent, more or less vestigial, former polyphagic habit. If 

 this latter explanation be the true one, it accounts for the varying 

 degree of potential polyphagy found in so many insects. Little 

 is known with respect to the host-preferences exercised by 

 different individuals of a true polyphagic species — whether they 

 do exhibit selection of the specific hosts upon which they them- 

 selves have developed in preference to all others. The beginnings 

 of oligophagy are initiated by preference of this kind, and the 

 more the oligophagic habit is developed, the more the species 

 loses its former trophic plasticity or power of accommodation to 

 new hosts, The repetition of the habit of specific host-preference 



