Aside from the class of adaptations that are for the good of the individual, there is another class connected solely with the preservation of the race. The organs for reproduction are the most important examples of this kind. These organs are of no use to the individual for maintaining its own existence, and, in fact, their presence may even be deleterious to the animal. The instincts connected with the use of these organs may lead inevitably to the death of the individual, as in the case of the California salmon, which, on entering fresh water in order to deposit its eggs, dies after performing this act. The presence of the organs of reproduction in the individual is obviously connected with the propagation of other individuals. Indeed in many organisms the life of the individual appears to have for its purpose the continuation of the race. In a large number of animals the individual dies after it has deposited its eggs. The most striking case is that of the May-flies, whose life, as mature individuals, may last for only a few hours. The eggs are set free by the bursting of the abdomen, and the insect dies. The male bee also dies after union with the queen. In some annelids, the body is also said to burst when the eggs are set free; and in other forms those parts of the body containing the eggs break off, and, after setting free the eggs, die. These are extreme cases of what is seen in many animals, namely the replacement of the old individuals by a new generation; and while in general there is only a loose connection between the death of the individual and the consummation of its reproductive power, yet the two run a course so nearly parallel that several writers have attempted to explain this connection as one of racial adaptation. It has also been pointed out that in those higher animals that take care of their young after birth, the life of the individual does not end with the period of birth of the young, but extends at least throughout the time necessary to care for the young. It has even been suggested that this lengthening of the life period has been acquired on account of its use to the species. When, however, as in the case of the vertebrates, the young are born at intervals either in great numbers at a birth, as in fishes and amphibia, or in lots of twos, threes, or fours, as in many birds and mammals, or even only one at a time, as in a few birds and in man, it will be evident that the relation cannot be so simple as has been supposed. It cannot be assumed in these forms that the end of the life of the individual is in any way connected with the ripening of the last eggs, for, on the contrary, hundreds, or even many thousands, of potential eggs may be present in the ovaries when the animal is overtaken by old age, and its power of reproduction lost.