CHAPTER XI 



THE PRACTICAL APPLICATION OF ECOLOGY— 



continued 



Host-selection and Biological Races, p. 286. The Adoption of 

 New Food-plants, p. 294 ; Hybridisation Experiments, p. 297 ; The 

 Interpretation of Host-selection, p. 297. Ecological Aspects of 

 THE Locust Problem, p. 299. The Phase Theory, p. 300 ; Factors 

 Governing Swarming, p. 305 ; The Species of True Locusts, p. 306 ; 

 Phase Development in Grasshoppers, p. 311. Literature, p. 313. 



Host-selection and Biological Races 



The last few years have witnessed the pubHcation of a number 

 of papers bearing upon the theory enunciated by Hopkins (1916) 

 and now generally known as the " host-selection principle." 

 According to this authority, in a given insect species that is capable 

 of breeding in two or more hosts, individuals will normally con- 

 tinue to select the particular host species upon which their own 

 life-cycle was passed. In other words, this host predetermines 

 selection by the ovipositing female of the same plant species for 

 her offspring as that in which her own developmental stages took 

 place. The outcome of the principle is the evolution of special 

 physiological strains, or races, each betraying its own particular 

 host-preferences, but, at the same time, exhibiting little or 

 no indications of morphological differentiation. This general 

 phenomenon was recognised more than sixty years ago by 

 Benjamin Walsh, who maintained that we have, in such host- 

 preferences, the beginnings of species differentiation. Such 

 preferences, he claimed, become habituated and destined through 

 heredity to result, in course of time, in increasing divergence and 

 to lead ultimately to the formation of separate species. 



Many cases of the apparent separation of polyphagous or 

 oligophagous species into different phytophagic or biological 

 races have been recorded {vide Thorpe, 1930). In some cases 



286 



