54 



[July, 



which, though abnormal, enables them to be of service to thu 

 creature. In the entire absence of connecting links, it requires a 

 considerable amount of faith to believe that these changes can 

 have been brought about by natural selection, especially as it 

 requires a liberal use of the imagination to conceive the steps by 

 which they could have been effected. 



The four posterior legs differ entirely from the anterior ; they 

 are short, compressed laterally, so as to resemble considerably the 

 fins of a fish ; and while in the Dyliscidce swimming is facilitated 

 • by the attachment of peculiar hairs to legs but slightly modified 

 from the ordinary type, in the Gyrinidce swimming hairs are also 

 present, but the entire leg is remarkably modified, and developed 

 into an organ exclusively suited for the purposes for which it is 

 destined. Moreover, in the Dytiscidce only the hind pair of legs 

 are specially modified, while in the Gyrinidce this is the case with 

 both the middle and hind pairs. 



Though the peculiar distinctness of the Gyrinidce as a group, and 

 the absence of anything like connecting links between them and other 

 beetles, would seem to be opposed to the idea of their being connected 

 by descent with other CoUoptera, yet the fact that within the bounds of 

 the group the species are very closely allied, yet variable, and that it is 

 not easy to fix with certainty the limits of some of the species, appears 

 to be favourable to the theory that all the Gyrinidce may have descended 

 originally from some one species. The difficulty above adverted 

 to of distinguishing the species of Gyrinus from one another is not 

 diminished by the fact that they are generally found in little colonies, and 

 that these colonies often consist of two or three species ; sometimes 

 the most allied species being found together, and at other times the 

 most dissimilar. 



We have in Britain two genera of this family ; they are very easily 

 distinguished by the following characters : — 



1. Body entirely destitute of pubescence, extremity of abdomen broad, 



and rounded at its apex — GxEiNrs. 



2. Body covered with a thick, short pubescence, extremity of abdomen 



conical — Oeectochilus. 



1. — HyRiycs, Geoffroy. 

 Our species of Gyrinus may be arranged in three groups — 



* Under surface entirely testaceous — G. minutus and urinator. 



