"One day, while feeding my pigeons, I noticed that the old wrens were greatly disturbed by something or other. They kept flying about me, uttering sharp, complaining cries; they would now and then fly to the outhouse, and then back to me. At last it occurred to me that some accident might have befallen the young wrens, so I proceeded to investigate, and soon discovered the trouble.
"Some one, in rummaging about the room, had overturned the boot, which had fallen in such manner that the top pressed against the wall, thus effectually barring the way to the nest. I righted the boot, thereby restoring the children to their parents, much to the delight of all parties concerned. Ever since this episode the male wren has shown his gratitude in an unmistakable manner. He has followed me into the house on several occasions; he has learned where I sit when engaged in sewing, and pays me short visits, flying though the window several times a day, and, wonderful to relate, after the young had learned to fly, he brought them around to my window and evidently gave them to understand that I was their saviour!"
The higher animals, such as the horse, the ox, the dog, the monkey, etc., show the emotions of anger, hate, fear, love, and grief so plainly that "he who runs may read." That these animals possess these emotions is a fact which hardly needs demonstration. They likewise have very retentive memories, sometimes treasuring up an injury for days, months, and years, until an opportunity arrives for them to "get even," thus showing that they are revengeful.
Thus, a dog of my acquaintance had been severely thrashed last winter by a larger dog. He bided his time, and, this summer, after his antagonist had been handicapped by having that atrocious invention, a muzzle, affixed to his head, he fell upon him, "tooth and toe-nail," and would have killed him had he not been prevented.
Again, some years ago my attention was called to a large mandril by the keeper of the monkey house in the St. Louis Zoölogical Garden, who remarked that "That monkey will do me up some day. I had to thrash him several days ago, and ever since then he has had it in for me."
Not ten minutes after the conversation, while I was in another part of the building, I heard a yell from the keeper, and, on rushing to see what had happened, found that the man's thumb had been almost severed from his hand by the powerful teeth of the mandril. The keeper had been explaining something to some visitors, standing with his back to the animal, and with his hand resting on one of the bars of the cage. The brute saw his opportunity, and, in the twinkling of an eye, seized it and inflicted a severe injury to the individual whom he regarded as his enemy.