454 SCIENCE PROGRESS 



and Mrs. Piper were both thinking of laurel wreaths. Perhaps 

 there was a Marathon race on at the time, perhaps there was 

 an article on laurels in the Daily Mail, perhaps they had been 

 reading classical poetry — a thousand suggestions occur to me 

 but I fail to see how the correct one would help us. In the 

 history of science we have often been struck with the number 

 of great discoveries which have been made simultaneously 

 by persons independently of one another, showing how the 

 thoughts of individuals are controlled by their times. Newton 

 and Leibnitz both thought of the differential calculus at the 

 same time ; Neptune was simultaneously discovered by Adams 

 and Leverrier, natural selection by Darwin and Wallace ; 

 Mendeleef and Lothar Meyer both hit upon the periodicity of 

 the properties of elements; the true functions of the semi- 

 circular canals in the ear occurred simultaneously to Mach, 

 Breuer and Crum Brown ; in 1904 two independent persons 

 thought of the possibility that ticks might carry the spirillum 

 of relapsing fever; and finally, in 1907 Mrs. Piper and Mrs. 

 Verrall were both thinking of laurel wreaths. Now all these 

 coincidences, except the last-named, are very interesting : a 

 valuable essay might be written on the social factors which 

 have brought about these simultaneous discoveries. But that 

 Mrs. Piper and Mrs. Verrall should both have been thinking of 

 laurel wreaths in February 1907 is neither interesting nor in 

 the least significant nor worth investigating whether it be true. 

 Why should they not both have been thinking of laurel wreaths ? 

 They must, I suppose, have been thinking of something or 

 other : the range of possible thoughts is not infinite — indeed 

 with most people it is singularly limited. It is tolerably certain, 

 a priori, that large numbers of people must always be thinking 

 of the same thing at the same moment. I cannot see, there- 

 fore, why anybody should be in the least disconcerted by the 

 information that Mrs. Piper and Mrs. Verrall were both thinking 

 of laurel wreaths in February 1907. Besides, perhaps they 

 were not ; mistakes will occur and it is much more likely that 

 one of these ladies made a mistake in trying to recall the subject 

 of her thoughts than that " psychical cross-correspondences " 

 are true. But, as I have already observed, there is no reason 

 to suppose that there was a mistake ; since the fact (if true) 

 bears no relation to the conclusion deduced from it by Sir Oliver. 

 In his Becquerel Memorial Lecture delivered before the 



