1 08 EVOL UTION AND NA TURAL THEO LOGY. 



opportunities for exercise, especially during 

 youth ; and subsequently to inheritance. On 

 the other hand, herbaceous plants growing on 

 islands where they have few or no competitors, 

 often tend to assume an arboreal habit.* 



Difference in the size of animals of the same 

 species does not always arise from one cause. 

 The insects of Trinidad are frequently smaller 

 than those of Guiana, which may arise from 

 Trinidad being an island ; but before concluding 

 that this is the correct explanation, it would be 

 necessary to compare the average size of speci- 

 mens from the opposite coast of Venezuela also. 

 Contrary to the popular impression, great heat 

 appears to have a tendency to reduce the size of 

 insects, perhaps because the larvae feed up more 

 rapidly in a hot climate. The tropical re- 

 presentatives of widely distributed genera are 

 frequently inferior in size and beauty to those 

 of temperate climates, and rarely surpass them, 

 for the magnificent productions of the tropics 

 usually belong to groups entirely unrepresented 

 in colder regions. In most cases where Indian 

 insects are also found in Europe or Japan, 



* Darwin, " Origin of Species," pp. 423, 424. 



