“No, that isn’t it; that’s the other kind of ivy, or woodbine, or creeper, as we call it,” replied Clinton.
“What is the difference?” inquired Whistler.
“A good deal of difference;—one is poisonous and the other isn’t,” said Clinton.
“I know that; but how do you tell one from the other?”
“You see the leaves grow in clusters?”
“Yes; there are five of them. Each leaf looks as if it were made up of five little ones.”
“Well, the leaves of the poison ivy have only three in a cluster; and that is the way I tell the difference between them. When the leaves grow in threes, look out; but when they are in fives, there’s no danger.”
“I must try to remember that,” said Whistler, repeating to himself the last remark of Clinton. “Let me see,—how can I fix that in my mind, so that I shall know ‘which is which,’ as they say? Now, I have it! If the leaf has five fingers, like my hand, I can handle it; if it hasn’t, I must not touch it.”
This process, in Whistler’s mind, was not a mere boyish whim. It was founded on a law planted deep in our mental natures,—the law of the association of ideas. It is difficult to remember a number or figure standing by itself; and the matter becomes still worse when two numbers are to be borne in mind, and distinguished from each other, as in this instance. But, by associating the number in the mind with some particular object, event or word, we have a clew to it, which will seldom fail us; and if the word, event or object, bears any resemblance to the number, it is all the better. Thus you see that Whistler was quite a philosopher in this matter, although he did not know it. By making the act of handling depend upon the fancied resemblance of the leaf to his hand, he would never be at a loss to tell whether it was the three or five-lobed leaf that he was to avoid.
“There’s a three-leaved one!—that’s a poison ivy, isn’t it?” exclaimed Whistler, a few moments after, pointing to a vine that looked very much like the other, except in the number of its leaflets.