PROTECTIVE RESEMBLANCES-DIMORPHISM, ETC. 47 



or one of them would quickly disappear. I -believe 

 that it is a benefit to the species that some of its 

 larvae should resemble brown and others green catkins, 

 instead of all of them resembling either brown or green. 

 In the former case the foes have a wider range of 

 objects for which they may mistake the larvse, and 

 the search must occupy more time, for equivalent 

 results, than in the case of other species which are 

 not dimorphic. 



Dimorphism is also valuable in another way : the 

 widening range of a species may carry it into coun- 

 tries in which one of its forms may be especially 

 well concealed, while in other countries the other 

 form may be more protected. Thus a dimorphic 

 species is more fully provided against emergencies 

 than one with only a single form. To take an ex- 

 ample : the green colour of the young caterpillars of 

 the Convolvulus Hawk Moth (Sphinx convolvuli) some- 

 times persists, and is sometimes replaced by brown 

 in the later stages. In Europe the latter form pre- 

 dominates, because the creeping food-plant (Con- 

 volvulus arvensis) is so small that it is safer for a large 

 caterpillar to resemble the earth beneath rather than 

 the small leaves on its surface. In the Canary Islands 

 and Madeira, where the larva feeds on many large- 

 leaved species of Convolvulus, the green form pre- 

 dominates, for it is far better protected than the other 

 against a continuous green background. 



This result appears to have been brought about by 



