Some of the lower Crustaceans are able to propagate their kind
sexually, and this will account for the extreme rarity of the males;
thus von Siebold (93. Beitrage zur Parthenogenesis, p. 174.)
carefully examined no less than 13,000 specimens of Apus from
twenty-one localities, and amongst these he found only 319 males. With
some other forms (as Tanais and Cypris), as Fritz Muller informs me,
there is reason to believe that the males are much shorter-lived than
the females; and this would explain their scarcity, supposing the two
sexes to be at first equal in number. On the other hand, Muller has
invariably taken far more males than females of the Diastylidae and of
Cypridina on the shores of Brazil: 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. According to the large experience of Mr. C. Spence
Bate, the reverse seems to be the case with six common British crabs,
the names of which he has given me.
THE PROPORTION OF THE SEXES IN RELATION TO NATURAL SELECTION.
There is reason to suspect that in some cases man has by selection
indirectly influenced his own sex-producing powers. Certain women tend
to produce during their whole lives more children of one sex than of
the other: and the same holds good of many animals, for instance, cows
and horses; thus Mr. Wright of Yeldersley House informs me that one of
his Arab mares, though put seven times to different horses, produced
seven fillies. Though I have very little evidence on this head, analogy
would lead to the belief, that the tendency to produce either sex would
be inherited like almost every other peculiarity, for instance, that of
producing twins; and concerning the above tendency a good authority,
Mr. J. Downing, has communicated to me facts which seem to prove that
this does occur in certain families of short-horn cattle. Col. Marshall
(94. The Todas, 1873, pp. 100, 111, 194, 196.) has recently found on
careful examination that the Todas, a hill-tribe of India, consist of
112 males and 84 females of all ages that is in a ratio of 133.3 males
to 100 females. The Todas, who are polyandrous in their marriages,
during former times invariably practised female infanticide; but this
practice has now been discontinued for a considerable period. Of the
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