100 THE WORLD OF LIFE CHAP, vi 



bringing about modification of specific forms ; and as the 

 climatal conditions throughout all these changes have 

 continued to be highly favourable to the support of 

 vegetation and of animal life, there has been a constant 

 tendency to produce and maintain an almost exact 

 equilibrium between the various species in the same area. 

 None being better adapted to the environment than a great 

 many others, none are able to monopolise large areas to the 

 exclusion of others, as is the case in the more changeable 

 temperate or cold regions. Whether we consider the differ- 

 ences between day and night temperatures, the variations of 

 temperature from month to month or from year to year, or 

 those extreme variations which we experience once perhaps 

 in a generation or in a century, such as excessively cold 

 winters, excessive droughts or excessive rains in summer, or 

 long periods of dry and cold winds all alike are unknown 

 in the equatorial regions, save in a few limited and quite 

 exceptional areas. In these more favoured portions of our 

 earth there prevails such a general approach to uniformity of 

 conditions (without ever reaching absolute uniformity) as 

 seems best adapted to bring about the greatest productivity, 

 together with extreme diversity in every department of the 

 great world of life. 



The large amount of diversity of species we have seen 

 to occur in single fields long subject to almost identical 

 conditions in our own country, with the additional fact that 

 no plot of a few square yards has exactly the same grouping 

 of species and individuals as any of the other plots, yet each 

 plot produces very nearly the same number of species, will 

 enable us in some degree to appreciate the conditions of the 

 tropics. There we see enormous areas subject to almost 

 identical conditions of soil, climate, and rainfall, yet in every 

 part of it exhibiting, amid a general uniformity of type, a 

 wonderful diversity in the shapes and structures of the forms of 

 life, and a no less wonderful balance and adaptation of each to 

 all. How this result has been actually brought about in the 

 course of evolution through the ages we shall better under- 

 stand after a brief exposition of the factors which have 

 been the immediate causes of the two great phenomena, 

 continuous evolution, with continuous adaptation. 



