very rapidly, and in some cases, as in evadne, produce eggs before they themselves are born. All their peculiarities are of such a character as to secure the greatest possible fertility; and thus to enable the animals to avail themselves, to the utmost, of the abundant supply of food.
Ramdohr found that a single isolated female daphnia produced 190 young in nineteen days, and he computed the number of descendents, at the end of sixty days, to be 1,291,370,075.
As the supply of food begins to fail in the fall, males are born, and the females produce the so-called winter eggs, which do not develop unless they are fertilized. These are few in numbers, much larger than the summer eggs, and they are incased in protecting shells. Their purpose is not to multiply the race, but to carry a few individuals through the winter, and over to the next season of plenty. They are slowly matured in the ovary, and contain an abundant supply of food yolk. They are not nourished in a broad chamber, and in many cases they have, in addition to the proper shell, an extra covering or ephipium, formed out of part of the integument of the parent. In daphnella three summer eggs are matured, at one time, in each ovary; but the animal produces only one winter egg, which is seven tenths as long as the whole body.
While the abundance or lack of food is a very important factor in determining the absence or presence of males, it is not the only one. Kurg found a few males in mid summer, but only in pools which were nearly dried up; and he was thus induced to attempt the artificial production of males. He was so successful that he obtained the males of forty species, in all of which the males had previously been unknown. He proved that any unfavorable change in the water causes the production of males, which appear as it dries up, as its chemical constitution changes, when it acquires an unfavorable temperature, or in general when there is a decrease in prosperity.
From these observations and from many others quoted by During, I think we may safely conclude that among animals and plants, as well as in mankind, a favorable environment causes an excess of female births, and an unfavorable environment an excess of male births.
Now, what is the reason for this law? If the welfare of the species can be secured, under a favorable environment, by females alone, why are males needed when the environment becomes unfavorable?
I have tried to show, in another place, from evidence of another kind, that the female is the conservative factor in reproduction, and that new variations are caused by the influence of the male. While the environment remains favorable no change is needed, but, as the conditions of life become unfavorable, variation becomes necessary to restore the adjustment, and I believe that we have, in Düring's results, an exhibition of one of the most wonderful and far-reaching of all the adaptations of Nature—an adaptation in virtue of which each organ-
