WHY WE MUST BE EVOLUTIONISTS
inner corner of our eye there is just a trace of what is called the third eyelid, a structure that is strongly developed and readily seen in most mammals, as well as in birds and reptiles. It serves to clean the front of the eye; but although it is big enough to do this in most mammals and birds it is a mere relic in man. Take another example: behind the eye of the skate—a familiar flat fish—there is a large hole called the “‘spiracle.” It serves for the incoming of the “breathing water,” which washes the gills and passes out by the five pairs of gill-clefts on the under surface. But if we peer into this very useful breathing-hole or spiracle we see a minute comb-like structure, which is the dwindling useless relic of a gill. The cleft or spiracle is of indispensable use to the skate, but the relic or vestigial gill inside the spiracle is of no use at all. Yet it tells us that a spiracle was evolved from a gill-bearing gill-cleft.
One of the most remarkable sets of facts about living creatures—plants as well as animals—is that old structures become transformed into things very new. The poet Goethe helped to make the great discovery that the parts of a flower—sepals, petals, stamens, and carpels—are just four whorls of transfigured leaves, the stamens and carpels being spore-bearing leaves. We sometimes see the whole flower of a flowering plant that has become too vegetative “go back” and become a tuft of green leaves; and it is an unforgettable lesson to pull the flower of the white water lily to pieces and to find that the green sepals pass gradually into white petals, and these gradually into yellow stamens.
Similar lessons are taught by animals. What is the sting of a bee but a transformed egg-laying organ or ovipositor (therefore never found in drones), and what is an ovipositor but a transformed pair of limbs? The elephant’s trunk was a great novelty in its way, but it is just a very long nose with
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