DEYELOPMEISTT OF TEOPICAL LIFE. 123 



changes experienced, being dependent on cosmical or 

 geological changes, were so slow, that variation and 

 natural selection were always able to keep the teeming 

 mass of organisms in nicely balanced harmony with the 

 changing physical conditions. The equatorial zone, in 

 short, exhibits to us the result of a comparatively con- 

 tinuous and unchecked development of organic forms ; 

 while in the temperate regions, there have been a series 

 of periodical checks and extinctions of a more or less 

 disastrous nature, necessitating the commencement of 

 the work of development in certain lines over and over 

 again. In the one, evolution has had a fair chance ; in 

 the other it has had countless difficulties thrown in its 

 way. The equatorial regions are then, as regards their 

 past and present life history, a more ancient world 

 than that represented by the temperate zones, a world 

 in which the laws which have governed the progressive 

 development of life have operated with comparatively 

 little check for countless ages, and have resulted in those 

 infinitely varied and beautiful forms — those wonderful 

 eccentricities of structure, of function, and of instinct — 

 that rich variety of colour, and that nicely balanced 

 harmony of relations — which delight and astonish us in 

 the animal productions of all tropical countries. 



