NATURAL, SELECTION 



gullible acceptance during the Romantic Period, nonetheless those phe- 

 nomena are real. He backs up his opinion with a very large number of 

 examples, all of which are well thought out, but many of which could be 

 explained away on the basis of coincidence. He believes, however, that 

 their effect is cumulative, and that, taken together, they establish adaptive 

 coloration as one of the main achievements of natural selection. It would 

 be very difficult to read his book without inclining to his viewpoint. 



SEXUAL SELECTION 



A second special feature of Darwin's theory, that of sexual selection, 

 has fared even less enviably than has that of adaptive resemblances. 

 Darwin believed that the general theory of natural selection could not 

 account for the color differences which so commonly characterize the 

 sexes, nor for other types of ornamentation differentiating the sexes. He 

 therefore proposed the theory of sexual selection to account for such differ- 

 ences. In brief, the theory is that the female selects her mate, and that 

 therefore any male which is especially attractive will have improved 

 chances of obtaining a mate and leaving descendants. This leads to de- 

 velopment, in the male, of brilliant colors, elaborate combs, ornamentative 

 hooks, in fact, of any secondary sex character which might be regarded as 

 potentially attractive. Darwin included also antlers, tusks, and spurs, 

 which, however, could also be accounted for by the general theory of 

 natural selection. 



The idea of sexual selection requires either that males be more numer- 

 ous than females, or that polygamy be the rule; that the ornamentation of 

 the males be attractive to the females; and that the females select their 

 mates. Some positive evidence in favor of the theory has been obtained, 

 but it is very scant. Generally speaking, the numbers of males and females 

 seem to be about equal. There are some outstanding examples of polyga- 

 mous societies among animals. One of the best is the fur seal, which breeds 

 on the Pribilof Islands near Alaska. The males arrive at the breeding 

 grounds before the females, and then engage in mortal combat, in which 

 most of the males are either killed or driven away from the breeding 

 grounds. Upon arrival of the females, each surviving male has an extensive 

 "harem." Another fact which has been interpreted as supporting the theory 

 concerns those instances in which there is a reversal of the usual pattern 

 of sex behavior. For example, phalaropes ( a kind of sandpiper ) are char- 

 acterized by brilliant plumage in the female and dull plumage in the male. 

 But the female does the courting, while the male builds the nest and 

 incubates the eggs. 



There is much doubt how much influence ornamentation may have 

 upon actual selection of mates. The male seals referred to above have 

 powerful tusks, but the females do not appear to have any choice of 

 mates : they simply accept the male which is at hand when they arrive on 

 the breeding grounds. In some cases, secondary sex characters appear to 

 excite sexual activity without actually influencing choice of mates. In 

 Drosophila, for example, mating is preceded by courtship behavior which 



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