Timaeus (92)

Tim.

Owing to these practices they have dragged their front limbs and their head down to the earth, and there planted them, because of their kinship therewith; and they have acquired elongated heads of every shape, according as their several revolutions have been distorted by disuse.

On this account also their race was made four-footed and many-footed, since God set more supports under the more foolish ones, so that they might be dragged down still more to the earth. And inasmuch as there was no longer any need of feet for the most foolish of these same creatures, which stretched with their whole body along the earth, the gods generated these footless and wriggling upon the earth. And the fourth kind, which lives in the water, came from the most utterly thoughtless and stupid of men, whom those that remolded them deemed no longer worthy even of pure respiration, seeing that they were unclean of soul through utter wickedness; wherefore in place of air, for refined and pure respiring, they thrust them into water, there to respire its turbid depths. Thence have come into being the tribe of fishes and of shellfish and all creatures of the waters, which have for their portion the extremest of all abodes in requital for the extremity of their witlessness. Thus, both then and now, living creatures keep passing into one another in all these ways, as they undergo transformation by the loss or by the gain of reason and unreason. And now at length we may say that our discourse concerning the Universe has reached its termination. For this our Cosmos has received the living creatures both mortal and immortal and been thereby fulfilled; it being itself a visible Living Creature embracing the visible creatures, a perceptible God made in the image of the Intelligible, most great and good and fair and perfect in its generation—even this one Heaven sole of its kind.[*]