58 AN INTRODUCTION TO SCIENCE 



"The moon changes to-night; we shall have a change in 

 weather." "How often does your moon change, dear 

 madam?" asks the man of science. "Once a week, of 

 course." "Well, you see I have adopted the metric 

 system. My moon changes ten times in a month, and 

 therefore, as this is just the end of the first week, my 

 weather can't change for at least another day." 



How are we to know that phenomena which appear to be 

 alike are alike in quality, and not merely alike in appear- 

 ance, or, in other words, how can we tell that the fact that 

 they are alike indicates that their likeness is due to the same 

 cause ? Fifteen years ago, when Dr. Gaskell announced his 

 theory of the origin of Vertebrates from a crustacean-like 

 ancestor, with the amazing inferences as to changes in the 

 functions of organs which such a hypothesis implies, I hap- 

 pened to visit one of the most eminent of living zoologists, 

 to whom I expounded the evidence upon which the theory 

 was based. " Gaskell has a fiendish ingenuity in collecting 

 coincidences," was the professor's comment. But what 

 higher praise can be bestowed upon any observer? It is his 

 business to collect coincidences, and then to postulate the 

 cause which determines that the observed phenomena coin- 

 cide. When he has found this, he is in a position to 

 formulate a "law." Yet anyone who pays attention to this 

 matter will learn that it is very dangerous to conclude that 

 because things coincide therefore they have a common 

 cause. It is mathematically expressed in the Law of 

 Chance, and yet in everyone's experience there has hap- 

 pened at some time or other so startling a coincidence that 

 no Law of Chance seems adequate to account for it. Here 

 is one which could hardly be devised in the fertile brain of 

 a Sherlock Holmes. The present President of the Royal 

 College of Surgeons of Edinburgh told the writer that some 

 time ago a woman was brought into his ward in the In- 

 firmary at Edinburgh shot in the breast by a bullet from a 

 revolver which some one was examining in a pawn-shop. 



