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 Mhller 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 Diastylidas 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 
sl 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 . — 
lu 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- 
pedes 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. 
72 See, on this subject, Mr. Pickard-Cambridge, as quoted in ‘ Quarterly 
Journal of Science,’ 1868, p. 429. 
