268 THE DESCENT OP MAN. 



minute shields or cell-like bodies, ■which exhibit beautiful chang- 

 ing colors; these are absent In the females, and in both sexes of 

 one species." It would, however, be extremely rash to conclude 

 that these curious organs serve to attract the females. I am in- 

 formed by Fritz Miiller, that in the female of a Brazilian species 

 of Gelasimus, the 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 chelse. In some species of the genus, prob- 

 ably in all, the sexes pair and inhabit the same burrow. They 

 are also, as we have seen, highly intelligent animals. From 

 these various considerations it seems probable that the male in 

 this species has become gaily ornamented in order to attract or 

 excite the female. 



It has just been stated that the male Gelasimus does not ac- 

 quire 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 differences between the sexes. 

 We shall hereafter find the same law prevailing throughout the 

 great sub-kingdom of the Vertebrata; and in all cases it is emi- 

 nently distinctive of characters which have been acquired through 

 sexual selection. Fritz Miiller" 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 dif- 

 ferently constructed from those of the female; whilst young, his 

 claspers resemble those of the female. 



Class, 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 conspicuous: thus the female 

 of Sparassus smaragdulus is dullish green, whilst the adult male 

 has the abdomen of a fine yellow, with three longitudinal stripes 

 of rich red. In certain species of Thomisus the sexes closely re- 

 semble each other, in others they differ much; and analogous 

 cases occur in many other genera. It is often difficult to say 

 which of the two sexes departs most from the ordinary coloration 



" Glaus, 'Die (reilebenden Copepoden,' 1853, s. 35. 

 " 'Pacts and Arguments,' &c., p. 79. 



M 'A History of the Spiders of Great Britain,' 1861-64. For the fol- 

 lowing facts, see pp. 77, 88, 102. 



