As far back as the first man with brains in his head, there was an ache to know why the sun shone, the stars twinkled, the winds blew, why harvests here were plentiful and there scant. The whole burden of witchcraft, of fetichism, of beliefs in voodoo, is a pathetic proof of this human longing to explain. What, after all, are superstitions but premature explanations that overstay their time? When men of thought get a glimpse of an interpretation really true, they are eager to prolong that glimpse until it becomes a survey whose due tests confirm and buttress a well grounded anticipation. This exploring process reminds us of what took place long ago when an architect of unexampled boldness first imagined a dome for a temple, and brought his dream to fulfilment. He began by rearing a single arch, fairly strong, yet hardly strong enough; a second arch arose to meet the first at their common crest; now, in mutual support both had a stability neither could display alone; at last when the wall had gone full circle it had a strength vastly greater than that of any part by itself. The long-admired arch had indeed become no more than an element to be joined with other arches to create a unit of an order distinctly higher. For ages the men who studied nature looked upon it as little changed since it left its Maker’s hand. Of infinite stimulus was the perception that it is a drama, not a tableau, which spreads itself before the eye. Speedily and with incomparable instruction it was traced how every actor in that drama had been molded by the part it had played in maintaining itself upon the stage of life. Every rival, parasite or foe, every stress of climate, was studied in its influence on food or frame, while the ever-threatened doom for irresponsiveness was the extinction which befell countless forms once masters of the earth. No hue of scale or feather, no barb or tusk, no curve of beak or note of song but served a purpose in the plot or advanced the action in some conflict to the death. When Darwin was confronted in plant or beast by an organ or a habit which puzzled him, he was wont to ask, What use can this have had? And seldom was the question asked in vain. He laid great stress on the directive worth of a well-considered theory. He tells us, “I am a firm believer that without speculation there is no good and original observation.” In a letter he remarks, “It is an old and firm conviction of mine that the naturalists who accumulate facts and make many partial generalizations are the _real_ benefactors of science. Those who merely accumulate facts I cannot very much respect.”