ENVmONMENT AMONG ANIMALS AND PLANTS 331 



upon the surface of the water. The two sets of leaves are as 

 utterly different in their appearance as it is possible for leaves 

 to be. Yet the effect of the external conditions upon the young 

 leaf-rudiment determines which of the kinds is to appear.' ^ 



Examples may also be found in the life-history of free-living 

 organisms. One is afforded by the common honey-bee. As is 

 well known, a queen bee differs markedly from a worker bee in 

 shape. Both queens and workers arise from fertihzed eggs ; 

 whether a queen or a worker develops from any one egg appears 

 to depend wholly on the environment — a larva that gives rise 

 to a queen receiving in the first place different and presumably 

 more nutritious food than that received by a larva giving rise 

 to a worker, and, in the second place, inhabiting a cell which 

 differs in size and shape from that inhabited by a worker larva. 

 Another is afforded by the hfe-history of the plant lice (Aphids). 

 At certain seasons of the year winged forms appear. It has long 

 been suspected that the appearance of winged forms depends on 

 some environmental stimulus. That this is so has been rendered 

 practically certain by the work of Shinji, who has shown that 

 aphids reared on plants watered with certain solutions are winged 

 almost without exception.- 



Among mammals the assumption of a winter coat by the 

 lemming, ptarmigan, and variable hare is a similar phenomenon. 

 Sir John Eoss has told how a Hudson Bay lemming was protected 

 from the cold on board his ship by keeping it in the cabin. It 

 retained its normal summer coat during the winter. On exposing 

 it in a cage on deck to a temperature of 30° below zero, the fur 

 on the cheeks and a patch on each shoulder became perfectly 

 white during the first night. After another day's exposure ' the 

 patches on each shoulder had extended considerably, and the 

 posterior part of the body and the flanks had turned a dirty 

 white. ... At the end of a week it was entirely white except in 

 a dark band across the shoulders prolonged posteriorly down 

 the middle of the back.' ^ 



A curious example of the importance of the environment 

 before birth is afforded by the difference between the mule and 

 the jennet. The former is the product of a cross between a stalhon 

 and a she-ass, the latter between a jackass and a mare. There 



' Lock, Bece.nl Progress, p. 317. - Shinji, 'Wing Development in Aphids', 



Biol. Bull., vol. XXXV. ^ Quoted by Vernon, loc. cit., p. 242. 



