266 PHILOSOPHY OF ZOOLOGY. 
cases, each species has a peculiar manner of arranging its 
eggs, as well as a choice of situation. In every case, how- 
ever, they are placed within the vivifying influence of the 
solar rays, and are hatched at the season of the year most 
advantageous for the growth and the comfort of the fry. 
In other oviparous animals, such as birds, before the eggs 
can be deposited, a house or nest must be constructed, often 
consisting of various materials, collected with great labour, 
and formed with exquisite neatness, which in a few species 
is lined with the down which they pull from their bodies. 
In all these cases, obedience to this imstinct is cheerfully 
complied with, however difficult, and any obstacle to prevent 
the execution of its purposes occasions pain. 
b. In each species securing a supply of surtable food for 
its offspring.—The simplest form in which this law ts ob- 
served, consists in the parent depositing its eggs on those 
substances which are to serve as food for the young when 
hatched. This is familiarly displayed in the case of the 
cabbage-butterfly, which deposites its eggs on the leaves 
which are afterwards to serve as the food of its caterpillar. 
In the case of the Oestrus equi, the eggs are deposited in 
such a situation, that circumstances are likely to occur, by 
which they shall be conveyed to a proper place for the issu- 
ing forth of the larva, and for its obtaining a suitable supply 
of food. The female insect attaches her eggs to those parts 
only of the horse which are most liable to be licked by the 
tongue, by which process they are conyeyed into the sto- 
mach. There, they are almost instantaneously hatched, the 
larva, known by the name of dots, adhere to the coats of the 
stomach by hooks. with which they are provided, obtain 
food from the juices by which the horse is nourished, and 
when mature pass out with the dung, to undergo the future 
changes of life. 
