122 TROPICAL NATURE, AND OTHER ESSAYS. 



colour are its most prominent features ; and these are 

 manifested in the highest degree in those equatorial 

 lands where the vegetation acquires its greatest beauty 

 and its fullest development. The causes of these 

 essentially tropical features are not to be found in the 

 comparatively simple influence of solar light and heat, 

 but rather in the uniformity and permanence with which 

 these and all other terrestrial conditions have acted ; 

 neither varying prejudicially throughout the year, nor 

 having undergone any important change for countless 

 past ages. While successive glacial periods have devas- 

 tated the temperate zones, and destroyed most of the 

 larger and more specialized forms which during more 

 favourable epochs had been developed, the equatorial 

 lands must always have remained thronged with life ; 

 and have been unintermittingly subject to those complex 

 influences of organism upon organism, which seem the 

 main agents in developing the greatest variety of forms 

 and filling up every vacant place in nature. A constant 

 struggle against the vicissitudes and recurring severities 

 of climate must always have restricted the range of 

 effective animal variation in the temperate and frigid 

 zones, and have checked all such developments of form 

 and colour as were in the least degree injurious in 

 themselves, or which co-existed with any constitutional 

 incapacity to resist great changes of temperature or other 

 unfavourable conditions. Such disadvantages were not 

 experienced in the equatorial zone. The struggle for 

 existence as against the forces of nature was there always 

 less severe, food was there more abundant and more 

 regularly supplied, shelter and concealment were at all 

 times more easily obtained ; and almost the only physical 



