INTRODUCTION. XV 
animals, it is through their integuments, which are soft and easily 
permeable, or on their surface, that the respiratory function is exercis- 
ed, as in vegetables. 
In many animals, the mode of generation is not known. Of others, 
fixed to solid bodies, like vegetables by their roots, the power of re- 
production seems to be by buds, or gemme, or by means of a separa= 
tion, which operates naturally or accidentally, of some parts of their 
bodies, in which are ultimately developed the organs which at first 
were wanting. In all other animals, there are organs specially des- 
tined to generation. These organs distinguish the males from the 
females. In the greater part of animals, the sexes are distinct and 
separate, in two different individuals of the same species; but in some 
classes, the individuals are at once males and females. In this case, 
these beings are termed androgynous. Sometimes, the individuals 
possess both sexes, like the greater number of vegetables, and they 
are then called hermaphrodites. 
The animals which have the sexes separate, differ, also, among 
themselves. Those are termed oviparous, in which the germ of the 
young individual is separated from the parent for a time before birth, 
under the form of anegg. Viviparous animals, on the contrary, are 
those in which the young are nourished in an organ, termed the 
uterus, and are not excluded from the mother. till they have taken the 
form which they afterwards preserve. 
Other modifications are noticed among the oviparous animals, or 
those which deposit eggs. In some, the egg is impregnated within 
the animal, and then the shell, or covering, is generally solid or corne- 
ous. In others, such as fishes, frogs, some insects, and many mol- 
lusca, the impregnation of the ovum does not take place till after 
extrusion. ‘Two remarkable circumstances have been further observ 
ed, among oviparous animals. The one is, that in some species the 
ova are not truly excluded, but hatched in the parent animal, who 
thus preserves the imperfect beings, till they have acquired the requi- 
site solidity for being deposited in a place adapted to their further deve- 
lopement. These species, which are met with in very different classes 
are termed ovo-viviparous. The other singular fact to be noticed in 
regard to oviparous animals is, that in a very great number of species, 
the young, when hatched, have neither the form, the structure, nor the 
manners of the parent animal; and many live in altogether a different 
medium. ‘These animals undergo, in the course of their limited ex- 
istence, many organic transformations, or successive metamorphoses, 
