INTRODUCTORY 23 



If we turn to a very different feature, their respective liability 

 to albinistic variation, we find ourselves in precisely similar 

 difficulty. Passer domesticus is a species in which individuals 

 more or less pied occur with especial frequency, but in P. mon- 

 tanus such variation is extremely rare if it occurs at all. The 

 writer of the section on Birds in the Royal Natural History 

 (III., 1894-5, P- 393) calls attention to this fact and remarks 

 that in that species he knows no such instance. 



The two species therefore, apart from any differences that we 

 can suppose to be related to their respective habits, are charac- 

 terised by small fixed distinctions in colour-markings, by a 

 striking difference in secondary sexual characters, and by a 

 difference in variability. In all these respects we can form 

 no surmise as to any economic reason why the one species 

 should be differentiated in the one way and the other in the other 

 way, and I believe it is mere self-deception which suggests the 

 hope that with fuller knowledge reasons of this nature would be 

 discovered. 



The two common British wasps, Vespa vulgaris and Vespa 

 germanica, are another pair of species closely allied although 

 sharply distinguished, which suggest similar reflexions. Both 

 usually make subterranean nests but of somewhat different 

 materials. F. vulgaris uses rotten wood from which the nest 

 derives a characteristic yellow colour, while F. germanica scrapes, 

 off the weathered surfaces of palings and other exposed timber, 

 material which is converted into the grey walls of the nest. The 

 stalk by which the nest is suspended (usually to a root) in the 

 case of germanica passes freely through a hole in the external 

 envelope, but vulgaris unites this external wall solidly to the 

 stalk. In bodily appearance and structure the two species are 

 so much alike that they have often been confounded even by 

 naturalists, and to the untrained observer they are quite indis- 

 tinguishable. There are nevertheless small points of difference 

 which almost though not quite always suffice to distinguish the 

 two forms. For example the yellow part of the sinus of the eyes 

 is emarginate in vulgaris but not emarginate in germanica. V. 

 vulgaris often has black spots on the tibiae while in germanica the 



