And God said,
Let the waters bring forth abundantly the moving creature that has life, and fowl that may fly above the earth in the open firmament of heaven.
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Basil the Great
4. It is not thus with us. Why? Because we incessantly move the ancient landmarks which our fathers have set. We encroach, we add house to house, field to field, to enrich ourselves at the expense of our neighbour. The great fish know the sojourning place that nature has assigned to them; they occupy the sea far from the haunts of men, where no islands lie, and where are no continents rising to confront them, because it has never been crossed and neither curiosity nor need has persuaded sailors to tempt it. The monsters that dwell in this sea are in size like high mountains, so witnesses who have seen tell us, and never cross their boundaries to ravage islands and seaboard towns. Thus each kind is as if it were stationed in towns, in villages, in an ancient country, and has for its dwelling place the regions of the sea which have been assigned to it.
Instances have, however, been known of migratory fish, who, as if common deliberation transported them into strange regions, all start on their march at a given sign. When the time marked for breeding arrives, they, as if awakened by a common law of nature, migrate from gulf to gulf, directing their course toward the North Sea. And at the epoch of their return you may see all these fish streaming like a torrent across the Propontis towards the Euxine Sea. Who puts them in marching array? Where is the prince's order? Has an edict affixed in the public place indicated to them their day of departure? Who serves them as a guide? See how the divine order embraces all and extends to the smallest object. A fish does not resist God's law, and we men cannot endure His precepts of salvation! Do not despise fish because they are dumb and quite unreasoning; rather fear lest, in your resistance to the disposition of the Creator, you have even less reason than they. Listen to the fish, who by their actions all but speak and say: it is for the perpetuation of our race that we undertake this long voyage. They have not the gift of reason, but they have the law of nature firmly seated within them, to show them what they have to do. Let us go, they say, to the North Sea. Its water is sweeter than that of the rest of the sea; for the sun does not remain long there, and its rays do not draw up all the drinkable portions. Even sea creatures love fresh water. Thus one often sees them enter into rivers and swim far up them from the sea. This is the reason which makes them prefer the Euxine Sea to other gulfs, as the most fit for breeding and for bringing up their young. When they have obtained their object the whole tribe returns home. Let us hear these dumb creatures tell us the reason. The Northern sea, they say, is shallow and its surface is exposed to the violence of the wind, and it has few shores and retreats. Thus the winds easily agitate it to its bottom and mingle the sands of its bed with its waves. Besides, it is cold in winter, filled as it is from all directions by large rivers. Wherefore after a moderate enjoyment of its waters, during the summer, when the winter comes they hasten to reach warmer depths and places heated by the sun, and after fleeing from the stormy tracts of the North, they seek a haven in less agitated seas.