by a great multitude of individuals of a certain species, or by different species--for example, a change of climate--operates to make this struggle for existence still more severe, and the result would be that those individuals of the same species which could best adapt themselves to their new condition would tend to be preserved, as would the different species inhabiting the same country which could best maintain the struggle against other species. The improvement in the structure of the animals takes place, under this process of natural selection, in the following manner: The best individuals being preserved, the organs of which they make most use in the struggle for existence undergo development and slight modifications, favorable to the preservation of the individual, and these modifications are transmitted to their offspring. Here there comes in play a kind of collateral aid to which is given the name of "sexual selection," which is defined as a form of selection depending "not on a struggle for existence in relation to other organic beings or to external conditions, but on a struggle between individuals of one sex, generally the males, for the possession of the other sex."[23] "The result," continues Mr. Darwin, "is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore, less rigorous than natural selection. Generally, the most vigorous males, those which are best fitted for their place in nature, will leave most progeny. But, in many cases, victory depends not so much on general vigor, as on having special weapons, confined to the male sex." As, by means of this warfare of sexual selection, the victor would always be allowed to breed, his courage and his special weapons of offense or defense, in their increased development, would descend to his offspring. Thus the improvement and modification induced by natural selection would be enhanced and transmitted by the sexual selection.[24]