310 TROPICAL NATURE 



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 devastated the temperate zones, and 

 destroyed most of the larger and more specialised forms which 

 during more favourable epochs had been developed, the equa- 

 torial 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 develop- 

 ments 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 ex- 

 perienced 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 ob- 

 tained; and almost the only physical changes experienced, 

 being dependent on cosmical or geological revolutions, were so 

 slow that variation and natural selection were always able to 

 keep the teeming mass of organisms in nicely balanced har- 

 mony with the changing physical conditions. The equatorial 

 zone, in short, exhibits to us the result of a comparatively 

 continuous 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 de- 

 velopment 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 progress- 

 ive development of life have operated with comparatively 



