ENVIEONMENT AMONG ANIMALS AND PLANTS 329 



glycerine, several alcohols, certain salts . . . are all harmful, 

 retarding and distorting the embryo to a greater or less extent.' ^ 

 Many experiments show the effect of differences in food, tempera- 

 ture, and humidity upon developing organisms. Agar carried out 

 some experiments on a small water-flea, Simocephalus. This 

 animal is enclosed in a kind of shell composed of two valves, 

 somewhat like a mussel. Normally the edges of the valves 

 nearly meet, so that if a section is cut across the animal trans- 

 versely the shape of the body enclosed in the valves is oval. 

 When the food was varied in a certain way. Agar found that 

 the edges of the valves were turned outwards so that the shape 

 of a transverse section was no longer oval but bell-like. He 

 also found that the length was reduced by exposure to a high 

 temperature.2 Experimenting with a beetle, Tower found that 

 both the colour and the colour pattern could be modified by 

 changes in temperature and humidity .^ Similarly Morgan showed 

 that a species of the fruit fly exhibited a peculiar formation of 

 the abdomen in the presence of moisture ; when raised in dry 

 conditions the abnormality disappeared.* Poulton obtained some 

 remarkable results when working with the larvae of moths. 

 ' Larvae surrounded by the leaves on which they fed, became, 

 in the majority of species, light brown or light grey in colour. 

 If, however, an abundance of twigs had been mixed with the 

 leaves of the food plant, they became dark in colour. The larvae 

 of the Peppered Moth afforded the most striking result of all, 

 for when reared among green leaves and shoots they became 

 bright green without exception, whilst in the presence of dark 

 brown twigs they nearly all assumed a corresponding colour.' ^ 



A Japanese experimenter exercised rats for 90 to 180 days ; 

 he found that this long-continued exercise markedly increased 

 the weight of the heart, kidneys, and other organs on an average 

 to about 20 per cent.^ A thickening has been observed in the 

 stomach of a gull fed on grain for a year and this change is said 

 to take place under natural conditions in the herring gull, which 

 feeds in winter on fish and in summer on grain. Conversely if 

 graminivorous birds are fed on a carnivorous diet, the gizzard 

 assumes the form of a carnivorous bird's stomach. Cuvier found 

 the length of the intestine of the wild boar is as 9 to 1 and of 



* Jenkinson, loc. cit., p. 132. ^ Agar, Phil. Traits., Series B, vol. cciii,p. 319 



^ Tower, Investigation of Evolution, pp. 168 ff. ■* Morgan, loc. cit., p. 39 



^ Vernon, loc. cit., p. 219. ' Thomson, Animal Life, p. 383. 



