The birds, which do not carry food to their young, and do not therefore marry, are armed with spurs for the purpose of fighting for the exclusive possession of the females, as cocks and quails.  It is certain that these weapons are not provided for their defence against other adversaries, because the females of these species are without this armour.  The final cause of this contest amongst the males seems to be, that the strongest and most active animal should propagate the species, which should thence become improved.

Another great want consists in the means of procuring food, which has diversified the forms of all species of animals.  Thus the nose of the swine has become hard for the purpose of turning up the soil in search of insects and of roots.  The trunk of the elephant is an elongation of the nose for the purpose of pulling down the branches of trees for his food, and for taking up water without bending his knees.  Beasts of prey have acquired strong jaws or talons.  Cattle have acquired a rough tongue and a rough palate to pull off the blades of grass, as cows and sheep.  Some birds have acquired harder beaks to crack nuts, as the parrot.  Others have acquired beaks adapted to break the harder seeds, as sparrows.  Others for the softer seeds of flowers, or the buds of trees, as the finches.  Other birds have acquired long beaks to penetrate the moister soils in search of insects or roots, as woodcocks; and others broad ones to filtrate the water of lakes, and to retain aquatic insects.  All which seem to have been gradually produced during many generations by the perpetual endeavour of the creatures to supply the want of food, and to have been delivered to their posterity with constant improvement of them for the purposes required.

The third great want amongst animals is that of security, which seems much to have diversified the forms of their bodies and the colour of them; these consist in the means of escaping other animals more powerful than themselves.  Hence some animals have acquired wings instead of legs, as the smaller birds, for the purpose of escape.  Others great length of fin, or of membrane, as the flying fish, and the bat.  Others great swiftness of foot, as the hare.  Others have acquired hard or armed shells, as the tortoise and the echinus marinus.

Mr. Osbeck, a pupil of Linnaeus, mentions the American frog fish, Lophius Histrio, which inhabits the large floating islands of sea-weed about the Cape of Good Hope, and has fulcra resembling leaves, that the fishes of prey may mistake it for the sea-weed, which it inhabits.  Voyage to China, p. 113.

The contrivances for the purposes of security extend even to vegetables, as is seen in the wonderful and various means of their concealing or defending their honey from insects, and their seeds from birds.  On the other hand swiftness of wing has been acquired by hawks and swallows to pursue their prey; and a proboscis of admirable structure has been acquired by the bee, the moth, and the humming bird, for the purpose of plundering the nectaries of flowers.  All which seem to have been formed by the original living filament, excited into action by the necessities of the creatures, which possess them, and on which their existence depends.