588 THE POPULAR SCIENCE MONTHLY. 



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 sub- 

 sist 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 cor- 

 responding 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, 

 lastened 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 ani- 

 mal 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 

 sufiiciently 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 imper- 

 fection 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 ustially 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 unfavor- 

 able conditions to which they have been exposed that they are unable 

 to free themselves from the remains of the egg-shell, so that the num- 

 ber 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 in- 

 creased many hundred per cent. It is no exaggeration to say that, as 

 compared with the artificial, the natural method of propagation among 



