THE DOCTRINE OF EVOLUTION 109 



certain range of temperature, moisture, and light ; they require 

 more or less oxygen ; and they need a certain amount and 

 quality of food. As a general rule slight changes in these 

 various elements of the environment can be borne without 

 injury; greater changes may cause death. There are a num- 

 ber of cases in the animal kingdom where modifications in 

 the organism seem to be directly connected with environ- 

 mental changes, though it is not always easy to say what fac- 

 tor in the environment is responsible for the change. Thus it 

 has been stated that horses tend to decrease in size in north- 

 ern latitudes, on islands, and on mountains. Most of the 

 animals on islands are smaller than their continental relatives. 

 De Varigny, in his Experimental Evolution, says that, " In 

 the Canary Islands the oxen of one of the smaller islands are 

 much smaller than those of the others, although all belong to 

 the same breed; and the horses are also smaller, and the 

 indigenous inhabitants are in the same case, although belong- 

 ing to a tall race." The same author also cites the experi- 

 ments of Karl Semper, who stated that if the common 

 pond-snail of Europe is kept in small volumes of water, 

 less than five or six liters (a little over four or five quarts), 

 the animals do not attain their usual size, but remain more 

 or less dwarfed. The differences between the geographical 

 varieties of the gray squirrel are given on page 406. Among 

 the insects the influence of food in the production of the 

 queen bee, and the influence of temperature in the produc- 

 tion of the different broods of one of the swallow-tail butter- 

 flies have already been commented upon. It has also been 

 found that colored bodies in the vicinity affect the colors of 

 the pupae of certain butterflies (Fig. 57). Such color changes 

 are due to the susceptibility of the larva to surrounding colors 

 during a quiescent period before pupation. The experiments 

 on Arte'mia, described on page 148, may also be. referred to in 

 this connection. 



