The eye does not always see what is in front of it. Indeed it might almost be said, it sees only what is back of it, in the mind. Whenever I have any particular subject in mind, every walk gives me new material. If I am thinking about tree-toads, I find tree-toads. If I am dwelling upon birds' nests, I find plenty of nests which otherwise I should have passed by. If bird-songs occupy me, I am bound to hear some new or peculiar note.
Every one has observed how, after he has made the acquaintance of a new word, that word is perpetually turning up in his reading, as if it had suddenly become the fashion. When you have a thing in mind, it is not long till you have it in hand. Torrey and Drummond, the botanists, were one day walking in the woods near West Point. "I have never yet found so and so," said Drummond, naming a rare kind of moss. "Find it anywhere," said Torrey, and stooped and picked it up at their feet. Thoreau could pick up arrow-heads with the same ease. Many people have the same quick eye for a four-leafed clover. I may say of myself without vanity, that I see birds with like ease. It is no effort, I cannot help it. Either my eye or my ear is on duty quite unbeknown to me. When I visit my friends, I leave a trail of birds behind me, as old Amphion left a plantation of trees wherever he sat down and played.
The scientific habit of mind leads a man to take into account all possible sources of error in such observations. The senses are all so easily deceived. People of undoubted veracity tell you of the strange things they have known to rain down, or of some strange bird or beast they have seen. But if you question them closely, you are pretty sure to find some flaw in the observation, or some link of evidence wanting. We are so apt to jump to conclusions; we take one or two steps in following up the evidence, and then leap to the result that seems to be indicated. If you find a trout in the milk, you may be justified in jumping to a conclusion not flattering to your milkman, but if you find angle-worms in the barrel of rain-water after a shower, you are not to conclude that therefore they rained down, as many people think they do.
Or if after a shower in summer you find the ground swarming with little toads, you are not to infer that the shower brought them down. I have frequently seen large numbers of little toads hopping about after a shower, but only in particular localities. Upon a small, gravelly hill in the highway along which I was in the habit of walking, I have seen them several seasons, but in no other place upon that road. Just why they come out on such occasions is a question; probably to get their jackets wet. There was a pond and marshy ground not far off where they doubtless hatched. Because the frogs are heard in the marshes in spring as soon as the ice and snow are gone, it is a popular belief that they hibernate in these places. But the two earliest frogs, I am convinced, pass the winter in the ground in the woods, and seek the marshes as soon as the frost and ice are gone. I have heard the hyla pipe in a feeble tentative manner in localities where the ground was free from frost, while the marshes near by were yet covered with solid ice; and in spring I have dug out another species from beneath the leaf mould in the woods. Both these species are properly land-frogs, and only take to the water to breed, returning again to the woods later in the season. The same is true of the tree-frog, which passes the winter in the ground or in hollow trees, and takes to the marshes in May to deposit its eggs. The common bullfrog and the pickerel frog doubtless pass the winter in the bed of ponds and streams. I think it is quite certain that hibernating animals in the ground do not freeze, though by no means beyond the reach of frost. The frogs, ants, and crickets are probably protected by some sort of acid which their bodies secrete, though this is only a guess of my own. The frog I dug out of the leaves one spring day, while the ground above and below him was frozen hard, was entirely free from frost, though his joints were apparently very stiff. A friend of mine in felling some trees in winter cut through a den of field crickets; the ground was frozen about their galleries, but the crickets themselves, though motionless, were free from frost. Cut the large, black tree ants out of a pine log in winter, and though apparently lifeless, they are not frozen.