5 When I consider the things lowest in the scale of nature, which are within our view, and which, though earthly, and feeble, and mortal, are still the works of God, I am lost in admiration of the Creator, who is so great, in the great works and no less great in the small. For the divine skill seen in the formation of all creatures in heaven and earth is always like itself, even in those things that differ from one another; for it is everywhere perfect, in the perfection which it gives to everything in its own kind.
We see each creature made not as a whole by itself, but in relation to the rest of the creation; so that the whole divine skill is displayed in the formation of each, arranging each in its proper place and order, and providing what is suitable for all, both separately and unitedly. See here, lowest in the scale, the animals which fly, and swim, and walk, and creep. These are mortal creatures, whose life, as it is written, "is as a vapor which appears for a little time." Each of these, according to the capacity of its kind, contributes the measure appointed in the goodness of the Creator to the completeness of the whole, so that the lowest partake in the good which the highest possess in a greater degree.
Show me, if you can, any animal, however despicable, whose soul hates its own flesh, and does not rather nourish and cherish it, by its vital motion minister to its growth and direct its activity, and exercise a sort of management over a little universe of its own, which it makes subservient to its own preservation. Even in the discipline of his own body by a rational being, who brings his body under, that earthly passion may not hinder his perception of wisdom, there is love for his own flesh, which he then reduces to obedience, which is its proper condition.
Indeed, you yourselves, although your heresy teaches you a fleshly abhorrence of the flesh, cannot help loving your own flesh, and caring for its safety and comfort, both by avoiding all injury from blows, and falls, and inclement weather, and by seeking for the means of keeping it in health. Thus the law of nature is too strong for your false doctrine.
Source: Reply to Faustus the Manichaean (New Advent)