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WORLD-POWER AND EVOLUTION 



there is a high percentage of crossing-over. This reaches a 

 minimum at about 71°, remains low till about 80", and then in- 

 creases rapidly at higher temperatures. An exposure of about 

 two days is necessary in order to produce the new forms. This 

 exposure is effective only when the eggs are in their earliest stages. 

 Plough's curve is significant in many ways. It is essentially 

 the same as that which Ewing has worked out for similar changes 



50° C 



Normal 



Highly 

 Variable 



S5° 

 % 



'iGUHE 25. The Relation of Temperature to Variability 

 in Drosophila. Adapted from Plough 



5 % 



10% 



15% 5 



in the insects known as aphids. At a temperature of GS^F this 

 creature produces the maximum number of wingless, non-repro- 

 ductive forms, while at lower and higher temperatures the number 

 of winged forms with fully developed sex characteristics increases. 

 Another similar curve has been found by Howell for the contrac- 

 tion of the muscles of a frog's leg, the contraction being strongest 

 at about 80 °F. In "Civilization and Climate" I have given a series 

 of similar curves which include (1) plants with a maximum growth 

 at about 85°, (2) infusoria which reproduce most rapidly at 

 about 83°, (3) crayfish which are most active at 74°, (4) factory 

 operatives who do the most work when the temperature for day 



