358 THE DESCENT OF MAN 



by I'ritz MuUer tliat in the female of a Brazilian species of 

 Grelasimus tlie whole body is of a nearly uniform grayish 

 brown. In the male the posterior part of the cephalo-thorax 

 is pure white, with the anterior part of a rich green, shading 

 into dark brown ; and it is remarkable that these colors are 

 liable to change in the course of a few minutes — the white 

 becoming dirty gray or even black, the green "losing much 

 of its brilliancy. ' ' It deserves especial notice that the males 

 do not acquire their bright colors until they become mature. 

 They appear to be much more numerous than the females; 

 they differ also in the larger size of their chelae. In some 

 species of the genus, probably in all, the sexes pair and 

 inhabit the same burrow. They are also, as we have seen, 

 highly intelligent animals. From these various considera- 

 tions it seems probable that the male in this species has 

 become gayly ornamented in order to attract or excite 

 the female. 



It has just been stated that the male Gelasimus does not 

 acquire his conspicuous colors until mature and nearly 

 ready to breed. This seems a general rule in the whole 

 class in respect to the many remarkable structural differ- 

 ences between the sexes. We shall hereafter find the same 

 law prevailing throughout the great sub-kingdom of the 

 Vertebra ta; and in all cases it is eminently distinctive 

 of characters which have been acquired through sexual 

 selection. Fritz Muller" gives some striking instances of 

 this law; thus the male sand-hopper (Orchestia) does not, 

 until nearly full grown, acquire his large claspers, which 

 are very differently constructed from those of the female; 

 while young, his claspers resemble those of the female. 



Glass, Arachnida (Spiders). — The sexes do not generally 

 differ much' in color, but the males are often darker than 

 the females, as may be seen in Mr. Blackwall's magnificent 

 work." In some species, however, the difference is con- 



" "Pacts and Arguments," etc., p. '79. 



'8 "A History of the Spiders of Great Britain," 1861-64. For the follow 

 ing facts, see pp. 11, 88, 102. 



