310 TROPICAL NATURE II! 
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 
