The passion for Nature is by no means a mere curiosity about her, or an itching to portray certain of her features; it lies deeper and is probably a form of, or closely related to, our religious instincts. When you go to Nature, bring us good science or else good literature, and not a mere inventory of what you have seen. One demonstrates, the other interprets. Observation is selective and detective. A real observation begets warmth and joy in the mind. To see things in detail as they lie about you and enumerate them is not observation; but to see the significant things, to seize the quick movement and gesture, to disentangle the threads of relation, to know the nerves that thrill from the cords that bind, or the typical and vital from the commonplace and mechanical—that is to be an observer. In Thoreau's "Walden" there is observation; in the Journals published since his death there is close and patient scrutiny, but only now and then anything that we care to know. Considering that Thoreau spent half of each day for upward of twenty years in the open air, bent upon spying out Nature's ways and doings, it is remarkable that he made so few real observations. Yet how closely he looked! He even saw that mysterious waving line which one may sometimes note in little running brooks. "I see stretched from side to side of this smooth brook where it is three or four feet wide what seems to indicate an invisible waving line, like a cobweb against which the water is heaped up a very little. This line is constantly swayed to and fro, as if by the current or wind, bellying forward here and there. I try repeatedly to catch and break it with my hand and let the water run free, but still to my surprise I clutch nothing but fluid, and the imaginary line keeps its place." A little closer scrutiny would have shown him that this waving water line was probably caused in some way by the meeting of two volumes or currents of water. The most novel and interesting observation I can now recall is his discovery of how the wild apple-tree in the pastures triumphs over the browsing cattle, namely, by hedging itself about by a dense thorny growth, keeping the cows at arm's length as it were, and then sending up a central shoot beyond their reach.