to the hot and the dry.

Hence if the fluid and the dry supply the material for all bodies,

it is reasonable that things the composition of which is due to the

fluid and the cold should have liquid for their medium [and, if they

are cold, they will exist in the cold], while that which is due to the

dry will be found in the dry. Thus trees grow not in water but on

dry land. But the same theory would relegate them to the water, on

account of their excess of dryness, just as it does the things that

are excessively fiery. They would migrate thither not on account of

its cold but owing to its fluidity.

Thus the natural character of the material of objects is of the same

nature as the region in which they exist; the liquid is found in

liquid, the dry on land, the warm in air. With regard, however, to

states of body, a cold situation has, on the other hand, a

beneficial effect on excess of heat, and a warm environment on

excess of cold, for the region reduces to a mean the excess in the

bodily condition. The regions appropriate to each material and the

revolutions of the seasons which all experience supply the means which

must be sought in order to correct such excesses; but, while states of

the body can be opposed in character to the environment, the

material of which it is composed can never be so. This, then, is a

sufficient explanation of why it is not owing to the heat in their

constitution that some animals are aquatic, others terrestrial, as

Empedocles maintains, and of why some possess lungs and others do not.



21



The explanation of the admission of air and respiration in those

animals in which a lung is found, and especially in those in which

it is full of blood, is to be found in the fact that it is of a spongy

nature and full of tubes, and that it is the most fully charged with

blood of all the visceral organs. All animals with a full-blooded lung