210 PHYSICAL BASIS OF HEEEDITY 



and wingless individuals may differ more strikingly than 

 do species of the same genus. The winged forms arising 

 from the wingless produce wingless forms again in the 

 next generation that may be identical with those from 

 which they came. It has long been believed that environ- 

 mental influences bring about these transitions in aphids, 

 but only recently has critical evidence been obtained. The 

 clearest evidence is that of Shinji, with the rose aphid. 

 By sticking twigs of the rose in sand and flooding the sand 

 with water containing substances in solution — a method 

 first suggested by W. T. Clarke — the fluid being drawn 

 up into the leaves is sucked out by the aphids on the leaves. 

 As the following table shows, young aphids reared pn the 



Winged Apterous 



lodividuals. Individuals 



AgNO, 51 



CuSo. 34 1 



HgCl, 31 6 



NiSO. 955 5 



SbCl, 41 5 



PbCl, 12 2 



SnCl 579 8 



ZnCl, 49 2 



Mg salts 840 9 



Sugar 365 160 



Alcohol 2 288 



Alum 3 34 



Acetic acid 67 



Na salts 2 1029 



Ca salts 1 433 



K salts 3 324 



Sr Saltsl 1 220 



Tannin 1 14 



Urea 5 153 



Water, distilled 394 



Water, tap and creek 17 461 



Peptone 15 



salts of the heavy metals as well as on magnesium salts 

 and sugar became winged, while those reared on the other 

 substances in this list remain apterous. Here we have 

 an excellent example of how in one environment a given 

 germ-plasm produces one result, and in another environ- 

 ment a different result without any intermediate forms. 



