c83 V. THE NATURE OF EVOLUTION 



of some pupae can be altered by the colour of background on which 

 the caterpillars are reared. 



Sumner (70) was able to change the tail length of rats bj' chang- 

 ing the temperature at which they were breeded. High temperatures 

 induced long tails and low temperatures short tails. In this connec- 

 tion it should be remembered that, as is well known, cold climates 

 reduce body surfaces such as ears, tail, and neck, resulting in the 

 development of a generally more compact build of body. This tendency 

 is particularly well exemplified by the size of ears in foxes as one 

 passes from warmer to colder climate. This kind of adaptation may 

 be valuable on the point of the thermal regulation of the body. 



It is said that the red primrose had red flowers if kept at a tem- 

 perature ranging from 15° to 20° C. A plant with the same genes but 

 reared at a temperature of 30° to 35°C w4th other environmental con- 

 ditions unchanged, produced white flowers. If a plant with white 

 flowers is brought into a room at 15° to 20°C the flowers that develop 

 later will be red. This phenomenon may be likewise significant in 

 the regulation of heat effect. 



Rabbits of a certain kind, when grow adult, have black hair on 

 such heat-losing surfaces as ears, tail, and the ends of extremities, 

 but the bandages to keep warm the part from where the black hair 

 has been removed, will cause white hair to grow, whereas black hair 

 will result on the back or the abdominal region when hair is cut to 

 cool the region. 



A certain case of the fruit-fly, Drosophila, is distinguished from 

 the normal by the fact that these are very few black bands on the 

 abdomen. When this race is reared on a rich supply of moist food, the 

 abdominal bands are almost completely absent in all individuals. The 

 same stock raised on scant, dry food exhibits normal banding of the 

 abdomen. 



Thus organisms can readily change their characters according tO' 

 the change of environment. Such a change is always reversible, and 

 remarkably enough the change mostly appear to be favourable for the 

 organism, at least not deleterious. The last mentioned example of the 

 fruit-fly may not be regarded as a favourable change, but at least 

 harmless. 



2. Dauermodifikation 



The reversible, adaptive change observed in higher organisms can 

 likewise be seen in unicellular organisms, such as bacteria, protozoa,, 

 and fungi. Especially with bacteria this phenomenon has been well 



