272 MR. A. D.'mTCHAEL ok SO^rE UN-DESCRTBED 



as well as the lateral margins ; while the hind margin of the 

 female in the neighbourhood of the bursa is straight and spikeless. 

 The object of this is of course manifest. There are considerable 

 differences in the epimera and yentral surface, &c., but these 

 differences are, after all, comparatively slight modifications. 

 "When we come to deal with the second species, G. dispar, we 

 find that the female is very like that of G. platygaster ; it is very 

 much smaller, and presents such well-marked specific differences 

 that no naturalist, however averse to species-making, would 

 think of considering them as identical ; but, on the other hand, 

 every one would admit that they were closely-allied species, and 

 this whether they regarded general appearance or minuter struc- 

 ture. On turning to the male, however, the case is entirely 

 different. It is utterly unlike both the male of G. platygaster 

 and its own female ; and I venture to think that no arachnolo- 

 gist who had not found them in coitu, would have supposed the 

 male and female to belong to the same species or genus, pro- 

 bably hardly to the same family or subfamily. The male is not 

 above half the length of the female, and, contrary to the ordinary 

 rule in the genus, its breadth is rather greater in proportion to 

 its length than in the female. The abdomen is broadest in front 

 andnarrowest behind, exactly contrary to what occurs in the female ; 

 the raised edge, the bifid or trifid lobes, the great spines or spikes 

 — all of which characteristics form the principal features both of its 

 female and of both sexes of the larger species — are entirely absent. 

 The great spines are replaced by a few minute points, and there 

 are not any hairs on the body ; again, the legs, instead of 

 being long and slender, as in the female, are short and thick ; 

 the two hind pairs are entirely hidden beneath the abdomen : and 

 indeed the whole creature seems quite different. It does appear 

 to me very strange in a genus where the males and females 

 generally have only moderately marked differences, that in two 

 species, the females of which are so closely allied, one should 

 haA^e a male resembling the female as nearly as is usual in the 

 genus, and the other should have a male so extremely dissimilar 

 When we look at the fact that the two species are found in the 

 same place, and apparently under precisely the same conditions 

 in all respects, the question arises what can have been the cause 

 of this remarkable variation, and to that question I confess I 

 cannot at present offer any satisfactory explanation ; it is difiicult 

 to understand how survival of the fittest can have produced it. 



