Because the female glow-worm is without wings, but the male is a winged insect. The female, therefore, is endowed with the power of displaying a phosphorescent light. The light is only visible by night, but it is, nevertheless, beautifully adapted for the purpose stated, because the _male is a night-flying insect_, and never ventures abroad by day. [Verse: "They that go down to the sea in great ships, that do business in great waters these see the works of the Lord, and his wonders in the deep."--PSALM CVII.] There exists some difference of opinion between naturalists upon the uses of the light of a glow-worm; there are some who doubt that it is exhibited to attract the flying insect. The objectors, however, offer no explanation of the luminous properties of the worm. Sir Charles Bell says the preponderance of the argument is decidedly in favour of the explanation we have given. 1129. _Why does not the iris of the fish's eye contract_? Because the diminished light in water is _never too strong for the retina_. 1130. _Why is the eye of the eel covered with a transparent horny covering?_ Because, as the eel lives in holes, and pushes its head into mud, and under stones, &c., it needed such a covering to _defend the eye_. 1131. _Why is the whale provided with an eye, having remarkably thick and strong coats?_ Because, when he is attacked by the sword-fish and the shark, he is almost helpless against his enemies, as they fix themselves upon his huge carcase. He therefore dives with them down to a depth where the pressure of the water is so great that they cannot bear it. The eye of the whale is expressly organised _to bear the immense pressure of extreme ocean depths_, without impairing the sight. 1132. _Why have fishes no eyelids?_ Because the water in which they swim keeps their eyes moist. Eyelids would therefore be _useless to them_. 1133. _Why have fishes the power of giving their eye-balls very sudden motion?_ Because, having no eyelids (such organs being unnecessary to keep their eyes moist), they still need the power of freeing their eyes from the contact of foreign matters; and this is secured to them by the power they have of giving the eyeball a very rapid motion, which causes reaction in the fluid surrounding it, and _sweeps the surface_. This motion may frequently be seen in the eyes of fishes, in glass globes. [Verse: "And God made the beast of the earth after his kind, and cattle after their kind, and everything that creepeth upon the earth after his kind: and God saw that it was good."--GENESIS I.] 1134. _Why is the lachrymal secretion of the horse's eye thick and glutinous?_ Because, as his eye is large, and constantly exposed to dust on journeys, it is provided with a _viscid secretion_, which cleanses the eye, and more instantly and securely removes the dust, than a _watery_ secretion would. 1135. _Why does the lower bill of the sea-crow project beyond the upper one?_