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Show 260 THE PRINCIPLES OF PAr..TlL never known the females of any species to arrive before their males. During one spring he shot thirty-nine males of Ray's wagtail (Budytes Raii) before I:e sa~v a single female. Mr. Gould has ascertained b~ d1s~ectio~t as he informs me, that male snipes nrnve m tlns country before the females. In the case of fish, at tl~e period when the salmon ascend our rivers, the males m large numbers are ready to breed before the females. So it apparently is with frogs and toads. Throughout the great class of insects the males almost always emerge from the pupal state before the other sex, so that they generally swarm for a time before any females can · be seen. 3 The cause of this difference between the males and females in their periods of arrival and maturity is sufficiently obvious. Those males :vhi~h annually first migrated into any country, or whiCh m the spring were first ready to breed, or '"ere the most eager, would leave the largest number of offspring; and these would tend to inherit similar instincts and constitutions. On the whole there can be no doubt that with almost all animals, in which the sexes are separate, there is a constantly recurrent struggle between the males for the possession of the females. Our difficulty in regard to sexual selection lies in understanding how it is that the males which conquer other males, or those which prove the most attractive to the females, leave a greater number of offspring to inherit their superiority than the beaten and less 3 Even with those of plants in which the sexes are separate, the male flowers are generally mature before the female. Many hermaphrodite plants are, as first shewn by C. K. Sprengel, dichogamous; that is, their male ancl female organs are not ready at the same time, so that they cannot be self-fertilised. Now with such plants the pollen is generally matUl'e in the same flower before the stigma, though there are some exceptional species in which the female organs are mature before the male. CIIAP. VIII. SEXUAL SELECTION. 2Gl attractive males. Unless this result followed, tlie characters which gave to certain males an advantage over others, could ~ot be perfected and augmented through sexual selection. vVhen the sexes exist in exactly equal numbers, the worst-endowed males will ultimately find females ( excepti~g where polygamy prevails), and leave as many offspnng, equally well fitted for their ge~eral habits of life, as the best-endowed males. From variOus. facts and considerations, I formerly inferred that With most animals, in which secondary sexual cl~aracters were well d~veloped, the males considerably exceed.ed the females m number; and this does hold good m some few cases. If the males were to the females as two to one, or as three to two, or even in a. somewhat lower ratio, the whole affair would be SImple ; for the better-armed or more attractive males ·would leave the largest number of o:ffsprinO', But aft" · · · o er I?vestigatmg, as far as possible, the numerical propor- ~wns o~ tl~e sexes, I do not believe that any great mequahty m number commonly exists. In most cases sexual. selection appears to have been · effective in the followmg manner. . :f:et us take any species, a bird for instance and diVI~e .the females in~a~iting a district into two ~qual boches. th~ one .co~s~stmg of the more vigorous and b~tter-nour1shed mdividuals, and the other of the less vigorous and healthy. The former, there can be little doubt, would b~ ~eady to ~reed in the spring before the others; and th1s IS the opmion of Mr. Jenner We' h 1 d . Ir, w o ms urmg many years carefully attended to th h b't f b' d Th e a I 8 o. 11" s. ere can also be no doubt that the most VIgorous, healthy, and best-nourished females would on nn av.erage succeed in rearing the largest number of offsprmg. The males, as we have seen, are generally ready to breed before the females; of the males the |