XXVI.  If this knowledge has come within your reach and you are familiar with these branches of science, look at the differences of plants also, up to the artistic fashion of the leaves, which is adapted both to give the utmost pleasure to the eye, and to be of the greatest advantage to the fruit.  Look too at the variety and lavish abundance of fruits, and most of all at the wondrous beauty of such as are most necessary.  And consider the power of roots, and juices, and flowers, and odours, not only so very sweet, but also serviceable as medicines; and the graces and qualities of colours; and again the costly value, and the brilliant transparency of precious stones.  Since nature has set before you all things as in an abundant banquet free to all, both the necessaries and the luxuries of life, in order that, if nothing else, you may at any rate know God by His benefits, and by your own sense of want be made wiser than you were.  Next, I pray you, traverse the length and breadth of earth, the common mother of all, and the gulfs of the sea bound together with one another and with the land, and the beautiful forests, and the rivers and springs abundant and perennial, not only of waters cold and fit for drinking, and on the surface of the earth; but also such as running beneath the earth, and flowing under caverns, are then forced out by a violent blast, and repelled, and then filled with heat by this violence of strife and repulsion, burst out by little and little wherever they get a chance, and hence supply our need of hot baths in many parts of the earth, and in conjunction with the cold give us a healing which is without cost and spontaneous.  Tell me how and whence are these things?  What is this great web unwrought by art?  These things are no less worthy of admiration, in respect of their mutual relations than when considered separately.