We are still groping our way among the causes which rule the determination of the sexes. Biology has only been able to throw a few scattered lights on the subject, and we possess only a few approximate data; which nevertheless are turned to account by the breeders of insects. We are still in the region of illusion and imperfect prognostics. But the Osmia knows what we do not. She is deeply versed in all physiological and anatomical knowledge, and in the faculty of creating children of either sex at will. These pretty bees, "with coppery skin and fleece of ruddy velvet," which establish their progeny in the hollow of a bramble stump, the cavity of a reed, or the winding staircase of an empty snail-shell, know the fixed and immutable genetic laws which we can only guess at, and are never mistaken. This marvellous prerogative the Osmia shares with a host of apiaries, in which the unequal development of the males and females requires an unequal provision of space and of nourishment for the future larvae. For the females, who exceed in point of size, huge cells and abundant provision; for the more puny males, narrow cells and a smaller ration of pollen and honey. Now the circumstances which are encountered by the Osmia, when, pressed by the necessities of spawning, she searches for a dwelling, are often fortuitous and incapable of modification; and in order to give each set of larvae the necessary space "she lays at will a male or a female egg, according to the conditions of space." In this marvellous study, which constitutes, with the history of the Cerceris, the finest masterpiece of experimental entomology, Fabre brilliantly establishes all the details of that curious law which in the Hymenoptera rules both the distribution and the succession of the sexes. In his artificial hives, in glass cylinders, he forces the Osmia to commence her spawning with the males, instead of beginning with the females as nature requires, since the insect is primarily preoccupied with the more important sex, that which ensures par excellence the perpetuation of the species. He even forces the whole swarm which buzzes about his work-tables, his books, his bottles, and apparatus, completely to change the order of its spawning. He shows finally that in the heart of the ovaries the egg of the Osmia has as yet no determined sex, and that it is only at the precise moment when the egg is on the point of emerging from the oviduct that it receives, AT THE WILL OF THE MOTHER, the mysterious, final, and inevitable imprint. But whence does the Osmia derive this, "distinct idea of the invisible"? Here again is one of those riddles of nature which Fabre declares himself quite incapable of solving. (8/17.)