62 THE POPULAR SCIENCE MONTHLY. 



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 de- 

 scended 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 de- 

 posited 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 devel- 

 opment 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 re- 

 marked to me, " Er ist eine reine Denkmaschine." It was the 

 possession of a superlative reasoning faculty that rendered Helm- 

 holtz 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 dis- 

 turb or distort the work of reason. 



First and foremost is the love of one's own observations and opin- 

 ions. 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 dispu- 

 tants 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 



