Reproduction and Growth 173 



a total life-span of fifteen years not unknown. Some fish live 

 much longer — figures of thirty years for the striped bass, 

 sixty years for the carp, are mentioned — and therefore spawn 

 a greater number of times. 



As for the number of eggs, it depends on the size of the 

 body cavity of the fish and on the diameter of the egg. Within 

 any species the diameter does not vary greatly, and small 

 females will therefore have fewer eggs than large females 

 of the same species. However, there is a much more signifi- 

 cant variation, of which the relative size of body cavity and 

 of tgg is only an expression. It is a variation found through- 

 out the animal kingdom, and it is dependent on the extent to 

 which the parent provides for the survival of its offspring. 



In the oyster, where fertilization is a matter of luck, and 

 where survival of the fertilized egg to maturity is pre- 

 carious, the number of eggs produced by each individual 

 at each spawning is in the millions. In the human being, 

 where parents give much more care not only to fertilization, 

 but also to the protection of the offspring until it matures, 

 only one child — normally — is produced at a time. 



The same principle holds for the fishes. The cod, with 

 floating eggs which drift in the open sea, may produce as 

 many as nine million j fertilization and survival are largely 

 a matter of chance. Trout, as we shall see in the next chap- 

 ter, prepare nests, choose mates, and provide for a certain 

 amount of postnatal care of the young: their eggs number in 

 the lower thousands. And in the guppy, where the young 

 remain in complete safety in the body of the mother until 

 they can fend for themselves, fifty at a time is considered a 

 good-sized family. The greater the parental care, the smaller 

 the number of eggs necessary. For preservation of the spe- 

 cies is the great object, and any form in which an average of 

 at least two offspring do not reach maturity for every pair of 

 parents is inexorably doomed to vanish from the face of the 

 earth. 



