Page:Popular Science Monthly Volume 11.djvu/606

588 provision of Nature, but it has been shown to fail signally in some cases. Certain flies lay their eggs upon decaying animal matter, in order that the maggots may find an abundant supply of proper food; but the flies are sometimes misled by the odor of a species of arum, and lay their eggs upon the leaves of the plant. The young, of course, perish as soon as they are hatched, for they are unable to subsist upon vegetable food. The presence of a certain odor stands in relation to the existence of putrid flesh, and there is in the fly a corresponding relation between the sensation caused by this odor and a tendency to deposit her eggs. That is, the relation between a certain sensation and a certain action is in harmony with another relation between two phenomena, external to the fly; but this adjustment is not quite perfect, since the odor of the arum-plant, which is not the right odor, does what nothing but the odor of putrid meat should do.

The trap-door spider makes for its dwelling a round hole in the ground, which it lines with silk and covers with a lid or trap-door, fastened with a hinge and lying even with the ground, and fitting so exactly that it is no easy matter to find the hole, even when the animal has just been seen to enter it. To render the deception still more perfect, the top of the door is sometimes covered with living mosses and lichens, which the spider is supposed to plant in this place; the whole apparatus is very wonderfully made, and we can hardly admire sufficiently the instinct which enables the animal to construct it. This instinct may lead to a great mistake, for a close observer of the habits of this animal—J. T. Moggidge—found a nest in sandy soil, where there was no vegetation. The lid had its usual cover of moss, although this failed to answer its purpose, for the little round spot of verdure made the nest very conspicuous instead of helping to hide it. The process of fish-culture furnishes a good illustration of the imperfection of such highly-important instincts as those concerned with the perpetuation of the species. It is found that if trout, white-fish, shad, or many other species, are allowed to lay their eggs in the natural manner, a very great proportion—usually much more than half—fail to be fertilized, and of the remainder many are destroyed by crowding and lack of fresh water; many more are buried by sediment, or carried away by the current, so that only a very few develop and give rise to young fish; and many of the young are so weakened by the unfavorable conditions to which they have been exposed that they are unable to free themselves from the remains of the egg-shell, so that the number hatched is very small indeed as compared with the number of eggs. To obviate this, the male and female fish are caught just before the time at which the eggs are deposited. These are pressed out of the body of the mother, artificially fertilized, and placed in proper hatching-boxes, and in this way the number of young is increased many hundred per cent. It is no exaggeration to say that, as compared with the artificial, the natural method of propagation among