Page:Popular Science Monthly Volume 35.djvu/381

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hundred or a thousand young fishes, that one of them fell a victim each minute to the enemies of the air or of the water. While the death-rate is vastly less than it was during embryonic life, it is great enough to put an end to the entire school in a single day, were it not for the fact that each time a bird swoops down upon the little fishes out of the air above, and each time that a predacious fish darts in among them out of the depths and carries off a victim, the survivors profit by the new experience, and become more alert and vigilant and better able to escape future danger. While it is not possible to give figures, there can be no doubt that the chance for long life increases by a high geometrical ratio with age. Among salt-water fishes the death-rate is enormous at first, but it grows less and less as the individuals grow older; and the natural death-rate of adult fishes is infinitesimal as compared with the death-rate of the young. A high birth-rate has its advantages, since it gives an opportunity for selection, and thus contributes to the maintenance and gradual evolution and improvement of the standard of the race. Each adult fish is a survivor, picked out or naturally selected from among thousands or even millions of less favored brothers and sisters; and while many of the accidents which overwhelm the eggs and young are of such a character that individual peculiarities count for nothing against them, we can not doubt that, on the whole, the alert and energetic and intelligent fishes are most likely to escape, and to grow up to maturity and to bear descendants. A high rate of increase does unquestionably aid evolution by selection, but the well-known fact that it is reduced in all species with low death rates shows that its primary and most important purpose is to compensate for the loss from accidents and diseases and enemies, and to insure the perpetuation of the species.

A young fish with a million brothers and sisters must, before it reaches sexual maturity, be in imminent peril of life a million times before it is able to reproduce its kind; and the million perils are so grouped that most of them face it at the beginning of its life, and grow less and less frequent as it becomes older. The perils of a fish may be compared to a pyramid which tapers from a broad base in infancy to a pointed apex in mature life, and each species must be made up of individuals of all ages in a similar numerical ratio to each other. The perils of each individual fish seem to be accidental, but their average for the entire species conforms to exact numerical laws, and the number which die during the first day, the second day, and so on, of their lives, must be about the same, season after season. During the slow process of evolution the birth-rate of each species has been so regulated by selection that, after the natural mortality has been provided for, there shall be enough survivors in each generation to maintain