THE ARTIFICIAL PROPAGATION OF SEA-FISHES. 361 



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 preda- 

 cious 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 com- 

 pared 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 con- 

 forms 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 



vol. xxxr. — 23* 



