THE PLASTICITY OF LIFE 205 



of on its own normal food-plant, though the reverse experi- 

 ment does not succeed. Every one knows how extraordin- 

 arily plants change in different soils, or under the influence 

 of manure, and it is to be suspected that some of the 

 hundreds of new parasitic worms that are continually being 

 described from new hosts are really old acquaintances 

 modified by a slightly different nutritive environment. 

 This suggests a line of experiment that is sure to lead to 

 interesting results. 



A third group of environmental influences is that of 

 the physical energies heat, light, and electricity. Heat 

 chiefly affects the rate of growth, promoting the formation 

 of nuclein-substances, and thus inducing cell-divisions. 

 A minute Infusorian (Stylonichia) studied by Maupas was 

 seen to divide once a day at a temperature of 7 to 10 C., 

 twiceat ioto i5,thriceat 15 to 20, four times at 20to24, 

 and five times at 24 to 27 C. ... It is probably for this 

 fundamental reason that some molluscs are twice as large 

 in the Tropics as they are in Europe. In general, we may 

 say that increased warmth hastens growth and develop- 

 ment : in some cases it favours reproductivity, especially 

 of an asexual sort. The green-flies or Aphides multiply 

 prodigiously and parthenogenetically on our rose-bushes 

 and pear-trees in summer months, under the combined 

 influence of food and warmth ; when food becomes scarcer 

 and the weather colder in autumn, males are born, partheno- 

 genesis ceases, ordinary sexual reproduction recurs. 



Cold retards growth and development. Thus Camerano 

 found that tadpoles of Rana muta remained tadpoles for 

 three or four years, when kept at a low temperature. The 

 period of development of the North Sea herring may be 

 doubled (extended to fifty days or more) by cooling 

 the water. The puzzle that the population of floating and 

 drifting marine animals is denser in the Arctic waters than 



