122 DARWINISM AND HUMAN LIFE 



the light, but add a little acid so that the solution 

 is no stronger than yooo tn * one P er cent., and 

 Gammarus swims towards the light. 



Remove one or two of the metals from sea- 

 water, keeping up the alkalinity, and the sea- 

 urchin egg develops into twins. Raising the 

 temperature a little often has the same result. 



Cold slows growth and development, yet the 

 population of floating and drifting animals is 

 denser in the Arctic waters than in the Tropics 

 a curious fact which Prof. Loeb explains by showing 

 that lowering the temperature greatly increases 

 the duration of life. There are more generations 

 living at the same time. Lowering the temperature 

 of the caterpillar box may be followed by curious 

 aberrations of colour in the moths and butterflies, 

 especially in the direction of melanism (Standfuss, 

 Fischer, and others). Prof. Poulton showed that 

 the caterpillars of the small tortoise-shell, for 

 instance, are for a short time so sensitive that 

 those in a white or gilded box have light or 

 golden pupse, while those from the dark box have 

 dark pupse. 



The influence of diet alone might form the subject 

 of a course of lectures. Take the simple but very 

 suggestive fact reported by Marchal that a scale- 

 insect, Lecanium corni, becomes L. robiniarum when 

 reared on Robinia pseudoacacia instead of on its 

 own normal food-plant, though the reverse ex- 

 periment does not succeed. Or consider one of 

 the most interesting of recent researches. Mr. 

 C. W. Beebe l caused the scarlet tanager (Piranga 

 erythromelas) and the bobolink (Dolichonyx oryzi- 

 vorus) to keep their breeding plumage through. 



i American Naturalist (1908), voL xlii. p. 34. 



