KEPKODUCTION AND HEREDITY 269 



generation has more hope of surviving, and the task of egg-laying 

 can be much more rapidly discharged when there is no need for 

 the two sexes to seek each other. But rapidity is in this case an 

 inestimable advantage, as only a few bright days are available, 

 and the coming of the frost soon terminates their life. 



Plant-lice, too, have two generations. In spring and summer 

 we find parthenogenetic females, usually furnished with two pairs 

 of wings. They are viviparous, and their young again develop 

 into females. This is repeated for several generations, until 

 finally ir> autumn the winged males and the wingless generations 

 of fertilizable females appear ; these lay eggs which hibernate 

 and produce in the next year a viviparous generation of females. 



Still more complicated is the reproductive process in the 

 notorious Phylloxera vastatdx, the destroyer of vineyards. Its 

 economic importance alone justifies its mention. 



Until the beginning of the 'sixties Phylloxera was unknown 

 in Europe. It was imported from America and became only too 

 quickly acclimatized. The devastations caused by these tiny 

 animals are enormous. In the course of eight years they destroyed 

 in France alone 75,000 hectares of vineyards and caused a loss to 

 the country of about 300,000,000, costing France more than 

 the unfortunate war of 1871. In Germany, too, the damage done 

 by Phylloxera in the districts of the Khine and other vineyards 

 is very considerable. For years the Government has bravely 

 endeavoured to proceed against this pernicious pest with all the 

 means provided by science, but has not so far succeeded in 

 exterminating it, though it has been found possible to prevent 

 Phylloxera from spreading further afield. 



But more interesting than its economic importance is to us 

 its mode of life. From the winter eggs that have been laid 

 under the bark, small wingless insects emerge in spring, which 

 ascend the stem and deposit on the leaves numerous heaps of 

 unfertilized eggs. The eggs of this first ' nurse '-generation 

 develop again into wingless females, and as this process is in 

 the course of the summer repeated about seven or eight times 

 we can imagine how enormous the multiplication of this terrible 

 plague must be. They pierce the vines with their sharp pro- 

 boscis and suck the sap. Beginning with August, there appear 

 in addition to these females other more slender and winged females 



