of true causes. There are still many morphologists who feel that they have somehow explained matters when, for example, they state that the human embryo has gill-clefts because man is descended from a fishlike ancestor. In reality such a statement is no explanation of the causation, any more than it would explain vegetable humus to say that it is due to vegetable matter deposited on the ground. Such assertions may be true, but the omission of all links between the initial cause and the terminal effect shows that the notion of causation is in its rudimentary stage. There are too many naturalists who have still to develop a just conception of cause and effect; and it is just this development that we must look for in connection with experimental work. The biologists especially ought to profit more than they do by the opportunities offered in physiological laboratories.
The reasoning faculty is our weak point. Of the late Prof. Helmholtz, his friend, the physiologist Carl Ludwig, once remarked to me, "Er ist eine reine Denkmaschine." It was the possession of a superlative reasoning faculty that rendered Helmholtz to many of us the foremost scientific man of his time. Most of us certainly find that, when we try to reason, our reasoning is disturbed by various personal factors, and, though we know that emotional factors must be eliminated from intellectual processes if our conclusions are to be sure, yet experience has taught us that logic in our practice is rarely divorced from all emotion. Sound reasoning involves the character of the individual. To train a naturalist, it is even more important to perfect his character than his intellect. For this reason no teacher can deal advantageously with more than a few students, because he must understand the individual characteristics, and give each man personal guidance, which necessarily is different for each student.
Let us consider some of the factors which are most apt to disturb or distort the work of reason.
First and foremost is the love of one's own observations and opinions. If it takes the form of pride, which leads us to be so careful that our opinions deserve trust, well and good; but if it is merely an excitable vanity, it lures us to disaster. Think of the innumerable controversies of science, and tell me how often have the disputants cared less to prove themselves right, than to ascertain the truth, be their own opinions right or wrong. What we strive for and, I fear, never attain is perfect indifference to the sources of an idea. It is almost impossible not to feel an undue interest in our own idea, yet such an interest inevitably leads to overvaluation of the evidence in favor of our idea, and undervaluation of the evidence against it. Let us, therefore, avoid polemics, and so avoid the temptation to search for proof of a personal theory, when we ought to search for the truth only. Never let a pupil
