Chap. VIII.] SEXUAL SELECTION. 307 



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 Tanis and Cypris) 

 there is reason to believe, as Fritz Miiller informs me, that the 

 male is much shorter-lived than the female, which, supposing 

 the two sexes to be at lirst 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 Diastylids and of Cypridina ; thus, with a 

 species in the latter genus, sixty-three specimens caught the 

 same day, included fifty-seven 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. 0. Spence Bate, with six 

 common British crabs, the names of which he has given me. 



On the Power 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 ferti- 

 lize the female, as with certain cirripedes and perhaps 

 certain fishes. An inequality between the sexes in these 

 cases might have been acquired through natural selection, 

 but from their rarity they need not here be further con- 

 sidered. In all ordinary cases an inequality would be no 

 advantage or disadvantage to certain individuals more 

 than to others; and therefore it could hardly have re- 

 sulted from natural selection. We must attribute the 

 inequality to the direct action of those unknown condi- 

 tions, which with mankind lead to the males being born 

 in a somewhat larger excess in certain countries than in 

 others, or which cause the proportion between the sexes 

 to differ slightly in legitimate and illegitimate births. 



