INTRODUCTION Ixix 



Achatinellidae (Mollusca), the Proterhinidae (Coleoptera) and many other insects, might 

 be considered due to the absence or slackness of those agencies, which elsewhere cause 

 individuals less fitted to the environment to become eliminated. But it is obvious 

 that the lack of definiteness of specific characters may be due to lack of time. When 

 we consider that some of the creatures, which show the greatest individual variation, 

 form a large part of the food-supply of the once extremely numerous insectivorous 

 birds, and therefore might be supposed to have undergone a very rigorous selection, 

 it would appear either that this selection has been singularly ineffective in producing 

 uniformity of appearance in a species, or that the very great variability must be itself 

 advantageous. It is interesting in this connection to compare the great variability of 

 the underside (i.e. the parts exposed when at rest) of the native Pyram&is with the 

 usual great constancy of those parts in the immigrant and allied P. cardiii and 

 P. atalanta, in explanation of which it may be urged that these, in their usual 

 localities, undergo a hibernation for many months of the year and requiring therefore 

 a more complete protection, when resting, have arrived at a constancy of markings 

 that suit their resting places. P. tammeamea, on the other hand, is active all the 

 year, and brood follows brood without any dormant period of the adult. Spiders of 

 species most liable to be found by birds are often beautifully protected by their 

 resemblance to their environment. 



Species which exhibit an extraordinarily wide range of variability, form an 

 interesting study in the islands, for we often find one or more distinct species so 

 closely allied to these as to lead to the suspicion that they actually have been derived 

 from the others. These probable offshoots from variable species are generally much 

 rarer and less widespread than the latter, and probably generally, if not always, differ 

 in their habits. In this case the causes, which have produced them, are similar to 

 those mentioned above of other insects. They have the appearance of being extreme 

 forms of the variable species, as though they had overpassed the limits of variability 

 and become distinct. Their constancy is probably correlated with uniformity of 

 habits. 



There are some striking examples of discontinuous variation amongst the island 

 insects, and the same have been noticed in the case of the Achatinellid Mollusca. 

 The di- and tri-morphism in the colour of the legs of certain species of Plagithniysus 

 is of a very conspicuous nature, owing to the large development of the femora in 

 proportion to the body, these beetles in a slight degree having a cricket-like ap- 

 pearance. Even more striking are some of the dimorphic forms of the endemic 

 Locustidae {Ba7iza — Brachyniietopa). There is no evidence to suggest from a 

 comparison with allied species that these variations are likely to lead to distinct 

 species, the different forms pairing indiscriminately and not differing in habits. In 

 the case of one species of Banza (blackbiirni) a very remarkable form of sexual 

 dimorphism is seen to be resulting from the two forms, but not a differentiation of 



