338 SCIENCE FROM AN EASY CHAIR 



under water. Gradually the only effective method of 

 dealing with the case has been established. The old 

 European vine-stocks or standards have been grubbed up 

 in all but the very choicest vineyards, and American vines 

 have been planted in their place. On to these have been 

 grafted cuttings of the local French vines, and they have 

 taken kindly to their new conditions. The produce of 

 the French vineyards is now greater than it has ever been. 

 It had fallen from an annual yield of 1,300,000,000 gallons 

 to 650,000,000 but in 1900 it had risen again to a yield 

 of more than 1,400,000,000 gallons. 



This history is a striking instance of the vast import- 

 ance to civilised communities of a knowledge and control 

 of even such minute living things as the plant-lice, and of 

 the extraordinarily large results which obscure living things 

 may produce. It must tend to convince reasonable men 

 of the importance of accurate knowledge as to living 

 things and of the necessity of expending public money in 

 constantly improving and extending that knowledge. 



An ingenious illustration of the enormous fecundity 

 of the plant-lice occurs to me as worth giving in conclusion. 

 The late Professor Huxley a careful and trustworthy 

 authority calculated that the produce of a single aphis 

 would, in the course of ten generations, supposing all the 

 individuals to survive, " contain more ponderable substance 

 than five hundred millions of stout men ; that is, more than 

 the whole population of China." And this calculation is 

 held by some authorities to be below rather than above 

 the mark I 



