Chap. VJII. SEXUAL SELECTION. 315 



and therefore appear to be the more numerous. This is actually the 

 case with a few species ; but he mentions several species in six 

 genera, in which the females appear to be much more numerous 

 than the males. 71 The small size of the males in comparison with 

 the females, which is sometimes carried to an extreme degree, and 

 their widely different appearance, may account in some instances 

 for their rarity in collections. 72 



Some of the lower Crustaceans are able to propagate their kind 

 asexually, and this will account for the extreme rarity of the males. 

 With some other forms (as with Tanais and Cypris) there is reason 

 to believe, as Fritz Muller informs me, that the male is much shorter- 

 lived than the female, which, supposing the two sexes to be at first 

 equal in number, would explain the scarcity of the males. On the 

 other hand this same naturalist has invariably taken, on the shores 

 of Brazil, far more males than females of the Diastylida? and of 

 Cypridina ; thus with a species in the latter genus, 63 specimens 

 caught the same day, included 57 males ; but he suggests that this 

 preponderance may be due to some unknown difference in the habits 

 of the two sexes. With one of the higher Brazilian crabs, namely 

 a Gelasimus, Fritz Muller found the males to be more numerous 

 than the females. The reverse seems to be the case, according to 

 the large experience of Mr. C. Spence Bate, with six common British 

 crabs, the names of which he has given me. 



On the Tower of Natural Selection to regulate the pro- 

 portional Numbers of the Sexes, and General Fertility. — 

 In some peculiar cases, an excess in the number of one 

 sex over the other might be a great advantage to a 

 species, as with the sterile females of social insects, or 

 with those animals in which more than one male is 

 requisite to fertilise the female, as with certain cirri- 

 pecles and perhaps certain fishes. An inequality be- 

 tween the sexes in these cases might have been acquired 

 through natural selection, but from their rarity they 

 need not here be further considered. In all ordinary 



71 Another great authority in this class, Prof. Thorell of Upsala (' On 

 European Spiders,' 1869-70, part i. p. 205) speaks as if female spiders were 

 generally commoner than the males. 



"- See, on this subject, Mr. Pickard-Cambridge, as quoted in 'Quarterly 

 Journal of Science,' 1868, p. 429. 



