CONDITIONS FOR DEVELOPMENT 287 



The white plumage of the ptarmigan in winter (Fig. 122), 

 and the white winter coat of the weasel and of other animals, 

 have been associated in our minds with the snows. But ex- 

 periments have shown that the change in the pigmentation 

 is brought about by a lowering of the temperature. And 

 observation has shown that it does not always coincide with 

 the appearance of snow. 



Another effect of temperature upon development is shown by 

 the experiments made on insects and other animals. There 



FIG. 123. Effect of temperature on development 



In the butterfly Vanessa levana prorsa the two broods have distinct patterns. By keep- 

 ing the eggs, larvae, and pupae of the spring brood at a low temperature, it has been 

 possible to make the imagos appear in the fall with exactly the same coloring as the 

 spring brood. This showed that the spring form differs from the summer form because 

 of the influence of the temperature 



are many species of butterflies that produce two broods each 

 year. The pupa survives the winter, and the adult emerges in 

 the. spring. The eggs laid shortly after give rise to a genera- 

 tion that reaches maturity in the summer. In many of such 

 species the spring form is often distinctly different from the 

 summer form in size, pigmentation, or pattern (see F"ig. 123). 



Experiments have shown that the so-called local races or varieties 

 of insects differ from each other chiefly, if not entirely, because of the 

 influence of temperature. 



These examples of the influence of temperature on the life 

 of animals illustrate the irritability of protoplasm in a way that is 

 somewhat different from our earlier studies, and they illustrate 

 a different kind of response. Instead of getting a contraction or 



