18 ZOOLOGY 



of the southeastern United States, which is multiple- 

 brooded. The early spring form (marcellus form) is the 

 smallest, and has the tail tipped with white; the summer 

 form (ajax) is the largest, and has the tail two-thirds 

 longer than the marcellus form. Other illustrations of 

 this seasonal difference are given by the imported Cabbage 

 Butterfly, the Gray-veined White and the Spring Azure. 

 This dissimilarity between the broods of different seasons 

 is known as seasonal dimorphism or polymorphism. The 

 cause of the difference between the different broods seems 

 to be the dissimilar climatic, especially thermic, condi- 

 tions under which they have developed. Mr. W. II. 

 Edwards in this country, and Weismann and others in 

 Europe, have shown that, when the summer form is kept 

 during development in a refrigerator, the butterfly has the 

 color of the winter form. 



Protective Resemblance and Mimicry. Every one who 

 has visited a natural history museum must have noticed 

 that the polar mammals are apt to be more or less white, 

 while those which live in tropical forests are dark. It is 

 easy to understand that these colors in their proper sur- 

 roundings make the animals which wear them hard to 

 see. This may be of advantage in enabling them to 

 escape the observation of their enemies, which are seeking 

 for them, or to avoid being seen by their prey as they ap- 

 proach. This general resemblance to their background is 

 seen even in some caterpillars, e.g. the tomato-worm, which 

 is colored so exactly like the leaf on which it feeds that it 

 is hard to find. Other caterpillars, belonging to the geo- 

 metrid moths, have the color of the twigs of the plant on 

 which they feed, and the resemblance is heightened by the 

 way they have of stiffening and standing out like a branch 



