From this table it will be seen that in June, the month when the birth-rate was smallest, the ratio of boys to each 100 girls was highest, and very much above the average for the whole year; while in March, the month when the birth-rate was greatest, the ratio of boys was smallest.
More than 0,000,000 births took place in the seven months when the ratio of boys was below the average for the year, and only 4,000,000 in the five months when it was above the average; and the table shows clearly that an increase in prosperity, as measured by the birth-rate, is accompanied by a decrease in the ratio of boy-births, and vice versa.
Among the lower animals, satisfactory statistics are wanting; but During states that, while domesticated animals are much more prolific than their wild allies, there is also a much greater preponderance of female births; that when animals are taken from a warm to a cold climate, the ratio of male births increases; and that leather-dealers say that they obtain most female skins from fertile countries where the pastures are rich, and most male skins from more barren regions; and he thinks we may safely conclude that the lower animals, as well as man, give birth to the greatest number of females when placed in a favorable environment, and to most males in an unfavorable environment.
An extreme instance is furnished by those animals which, during the seasons when food is abundant, lose the power to copulate and multiply parthenogenetically at a marvelous rate of increase, giving birth to generation after generation of parthenogenetic females, so long as the environment remains favorable, but giving birth, as soon as the conditions of life become less favorable, to males and to females which require fertilization.
The cladocera and aphides furnish the most striking instances of this kind of parthenogenesis, which has apparently been acquired, not to secure fertilization, but to enable the animals to utilize to the utmost the conditions which are most favorable to them, and to expand and contract their numbers in conformity to changes in their environment.
Among the parthenogenetic cladoceras both males and females are to be found in the fall, and a few males are found in the early spring; but during the warm months of spring and summer only females are found. These multiply very rapidly through the summer by parthenogenesis, generation after generation, and they differ from the females which are fertilized by a male in many features, all of which are of such a character as to render the parthenogenetic females unusually fertile.
They produce small eggs, which are discharged from the ovary while immature, and are nourished in a vascular broad pouch. They have little or no yolk; they "are not protected by a hard shell, and they develop immediately into parthenogenetic females, which mature
