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
They received the command to fly above the earth because earth provides them with nourishment. In the firmament of heaven, that is to say, as we have said before, in that part of the air called οὐρανός, heaven, from the word ὁ ρᾶν, which means to see; called firmament, because the air which extends over our heads, compared to the æther, has greater density, and is thickened by the vapours which exhale from the earth. You have then heaven adorned, earth beautified, the sea peopled with its own creatures, the air filled with birds which scour it in every direction. Studious listener, think of all these creations which God has drawn out of nothing, think of all those which my speech has left out, to avoid tediousness, and not to exceed my limits; recognise everywhere the wisdom of God; never cease to wonder, and, through every creature, to glorify the Creator.
There are some kinds of birds which live by night in the midst of darkness; others which fly by day in full light. Bats, owls, night-ravens are birds of night: if by chance you cannot sleep, reflect on these nocturnal birds and their peculiarities and glorify their Maker. How is it that the nightingale is always awake when sitting on her eggs, passing the night in a continual melody? How is it that one animal, the bat, is at the same time quadruped and fowl? That it is the only one of the birds to have teeth? That it is viviparous like quadrupeds, and traverses the air, raising itself not upon wings, but upon a kind of membrane? What natural love bats have for each other! How they interlace like a chain and hang the one upon the other! A very rare spectacle among men, who for the greater part prefer individual and private life to the union of common life. Have not those who give themselves up to vain science the eyes of owls? The sight of the owl, piercing during the night time, is dazzled by the splendour of the sun; thus the intelligence of these men, so keen to contemplate vanities, is blind in presence of the true light.
During the day, also, how easy it is for you to admire the Creator everywhere! See how the domestic cock calls you to work with his shrill cry, and how, forerunner of the sun, and early as the traveller, he sends forth labourers to the harvest! What vigilance in geese! With what sagacity they divine secret dangers! Did they not once upon a time save the imperial city? When enemies were advancing by subterranean passages to possess themselves of the capitol of Rome, did not geese announce the danger? Is there any kind of bird whose nature offers nothing for our admiration? Who announces to the vultures that there will be carnage when men march in battle array against one another? You may see flocks of vultures following armies and calculating the result of warlike preparations; a calculation very nearly approaching to human reasoning. How can I describe to you the fearful invasions of locusts, which rise everywhere at a given signal, and pitch their camps all over a country? They do not attack crops until they have received the divine command. Or shall I describe how the remedy for this curse, the thrush, follows them with its insatiable appetite, and the devouring nature that the loving God has given it in His kindness for men? How does the grasshopper modulate its song? Why is it more melodious at midday owing to the air that it breathes in dilating its chest?
But it appears to me that in wishing to describe the marvels of winged creatures, I remain further behind than I should if my feet had tried to match the rapidity of their flight. When you see bees, wasps, in short all those flying creatures called insects, because they have an incision all around, reflect that they have neither respiration nor lungs, and that they are supported by air through all parts of their bodies. Thus they perish, if they are covered with oil, because it stops up their pores. Wash them with vinegar, the pores reopen and the animal returns to life. Our God has created nothing unnecessarily and has omitted nothing that is necessary. If now you cast your eyes upon aquatic creatures, you will find that their organization is quite different. Their feet are not split like those of the crow, nor hooked like those of the carnivora, but large and membraneous; therefore they can easily swim, pushing the water with the membranes of their feet as with oars. Notice how the swan plunges his neck into the depths of the water to draw his food from it, and you will understand the wisdom of the Creator in giving this creature a neck longer than his feet, so that he may throw it like a line, and take the food hidden at the bottom of the water.