72 INTRODUCTION TO SCIENCE 



these appendages did not react to a strong odour, 

 whereas here the control experiment comes in 

 in exactly the same conditions and to the same 

 stimulus another crayfish with its antennules 

 intact did actively respond. Pursuing precisely 

 the same two methods, the investigator proved 

 that the seat of smell was in peculiarly shaped 

 bristles on the outer fork of the antennules. 



A great experimental philosopher is reported 

 to have said: "Show me the scientific man who 

 never made a mistake, and I will show you one 

 who never made a discovery." This was in allu- 

 sion to the everyday method of "trial and error," 

 which is part of the logic of experimenting. 

 Different hypotheses are tried till the one that 

 fits the facts is found. 



It is interesting to notice that a scientific con- 

 clusion may sometimes be safely accepted before 

 its demonstration is visibly complete, a famous 

 instance being Harvey's demonstration of the 

 circulation of the blood (1628). From the struc- 

 ture of the heart, the observed flow in different 

 parts of the system, and the valves in the veins, he 

 almost completely demonstrated the circulation. 

 Only one step was awanting. "Harvey's diffi- 

 culty lay in the circumstance that as the micro- 

 scope was not in use, no known path existed by 

 which the blood could be conveyed from the 

 smallest arteries into the smallest veins; there was 



