In addition to that form of Sexual Selection where the female chooses a mate from among a number of competing males, and which may be designated _preferential mating_, there is another form of selection in which the males fight with one another for the mastery and the possession of the females. Among the higher mammals it is a very general fact that the males fight together for the possession of the females. This leads, especially in polygamous animals, to the better armed or stronger males becoming the parents of the next generation, which inherits the peculiarities of the parents. Thus the offensive weapons and the vigor of the males are continually increased, resulting in the antlers of the stag, the tusks of the boar, the fighting instinct and spurs of the gamecock, and the horns and strength of the bull. Even mammals that are not specially armed fight to the death for the possession of the females, such as beavers, moles, squirrels, and hares. Almost all male birds are especially pugnacious during the breeding season. Battles have been observed in such different groups as ducks, finches, woodpeckers, humming birds, and waders. Among fishes deadly battles occur between the males of sticklebacks. Also the males of salmons engage in deadly contests; among reptiles fighting occurs among the male tortoises, crocodiles, and lizards. Spiders and many butterflies often fight for the females. Thus Sexual Selection through the _law of battle_ occurs widely throughout the animal kingdom. This form of Natural Selection greatly increases the vigor and fighting power of male animals; for, in every case, the weaker males are either driven away, killed or wounded, and the field is left to the most vigorous for procreating their kind. The male stickleback is a little fish that builds its nest among the weeds, weaving the material together by a secretion from its kidneys. It is a very passionate little animal, and is exceedingly pugnacious in relation to its male rivals. The battles of the males are often very desperate. The combatants fasten tight to each other for a time, tumbling over and over again, until they appear to be completely exhausted. The males of the rough-tailed stickleback, when fighting, swim round and round one another, endeavoring to pierce each other with their raised lateral spines. In fighting they are perfect little furies, and their bite is very severe. Their lateral spines are used with such fatal effect during a battle that a male has been observed to rip open his opponent, so that the latter sank to the bottom in a dying condition. The females are very peaceful. When they come out of their hiding-place and view the nest that the male has made he is mad with delight.