Page:Popular Science Monthly Volume 29.djvu/127

Rh each case the number of turbot in the sea next year would be double what it is this; the year after that there would be four times as many; the next year eight times again; and so on in a regular arithmetical progression. In a very few decades the whole sea would become one living mass of solid turbot. As a matter of fact, since the number of individuals in any given species remains on the average exactly constant, we may lay it down as a general rule that only two young usually survive to maturity out of all those born or laid by a single pair of parents. All the rest are simply produced in order to provide for the necessary loss in infant mortality. The turbot lays fourteen million eggs, well knowing that thirteen million nine hundred and ninety-nine thousand nine hundred and ninety-nine will be eaten up in the state of spawn or devoured by enemies in helpless infancy, or drifted out to sea and hopelessly lost, or otherwise somehow unaccounted for. The fewer the casualties to which a race is exposed the smaller the number of eggs or young which it needs to produce in order to cover the necessary losses.

In fish generally it takes at least a hundred thousand eggs each year to keep up the average of the species. In frogs and other amphibians, a few hundred are amply sufficient. Reptiles often lay only a much smaller number. In birds, which hatch their own eggs and feed their young, from ten to two eggs per annum are quite sufficient to replenish the earth. Among mammals, three or four at a birth is a rare number, and many of the larger sorts produce one calf or foal at a time only. In the human race at large, a total of five or six children for each married couple during a whole lifetime makes up sufficiently for infant mortality and all other sources of loss, though among utter savages a far higher rate is usually necessary. In England, an average of four and a half children to each family suffices to keep the population stationary; above that number it begins to increase, and has to find an outlet in emigration. If every family had four children, and every child grew up to maturity and married, the population would exactly double in every generation. Even making allowances for necessary deaths and celibacy, however, I believe that as sanitation improves and needless infant mortality is done away with, the human race will finally come to a state of equilibrium with an average of three children to each household. But this is getting very far away indeed from the habits of flat-fishes.—Cornhill Magazine.

 

SKETCH of Francis Galton may appear with manifest fitness in the same number of the "Monthly" in which is published an abstract review of M. de Candolle's researches into heredity and the other conditions favorable to the production of men of science. Mr.