328 THE POPULAR SCIENCE MONTHLY. 



YBTj rapidly, and in some cases, as in evadne, produce eggs before 

 they themselves are born. All their peculiarities are of such a char- 

 acter 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 descend- 

 ents, 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 bom, and 

 the females produce the so-called icinter 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 ephi- 

 pium, 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 artifi- 

 cial 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 imfavorable tem- 

 perature, 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 enmronmeyit 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 Diiring'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- 



