FACTORS OF ORGANIC EVOLUTION o/ 



that the average breadth of these crabs had rapidly 

 diminished. Now Professor Weldon observed that, 

 during this period, a huge breakwater was being 

 built in Plymouth Sound, which affected the move- 

 ment of the water, and permitted the large quantities 

 of china clay which the rivers carry from Dartmoor 

 to the Sound, to be deposited in hitherto unprece- 

 dented amount in the Sound itself. Other foreign 

 matter also found its way into the Sound in increas- 

 ing degree, owing to the increase in population on its 

 banks. The suggestion was that a relative narrow- 

 ness of the shell became an advantage — a survival- 

 factor — to the crabs placed in this changing environ- 

 ment. Therefore Professor "Weldon made a simple 

 experiment. He placed a number of crabs in a 

 vessel filled with sea-water, and suspended some 

 fine china clay therein. When he came to measure 

 the living and the dead crabs, he found that the 

 former had, on the average, much narrower shells. 

 He further showed that the crabs with narrower 

 shells are able, more efficiently than the others, to 

 filter the water which passes through their gills. 

 The evidence is not absolutely conclusive, and 

 it will be necessary to make further measure- 

 ments of these crabs every few years ; but, at any 

 rate, these studies may be regarded as very nearly 

 tantamount to experimental proof of the theory of 

 natural selection. 



This I have selected as one representative in- 

 stance of the kind of evidence which is now being 

 accumulated. Other instances deal with mice and 

 sparrows and man and other creatures. For more 

 detail than I have space to recount I would refer 



