96 THE WORLD OF LIFE CHAP. 



and is therefore liable to be masked by other favourable 

 or adverse conditions of the environment, it yet makes itself 

 visible in every continent ; and in the comparison between 

 the north or mid-temperate and the tropical zones is so pro- 

 nounced that in fairly comparable areas the tropical species 

 are often (and probably on the average) double those of 

 the temperate zones. This seems to be the case among the 

 higher animals, as well as among all the vascular plants. 



Now all this is indicative of long and minute adjustment 

 to the special inorganic as well as the organic conditions ; 

 and the reason why the tropics as a whole far surpass the 

 temperate zones in the number of their specific forms, is, not 

 the greater amount of heat alone, but rather the much greater 

 uniformity of climatical conditions generally, during long 

 periods perhaps during the whole range of geological time. 

 Whatever changes have occurred through astronomical 

 causes, such as greater excentricity of the earth's orbit, 

 must necessarily have produced extremes of climate towards 

 the poles, while the equatorial regions would remain almost 

 unaffected, except by a slight and very slow rise or fall of 

 the average temperature, which we know to be of little im- 

 portance to vegetation so long as other conditions remain 

 tolerably uniform and favourable. 



It is this long-continued uniformity of favourable con- 

 ditions within the tropics, or more properly within the great 

 equatorial belt about 2000 miles in width, that has permitted 

 and greatly favoured ever-increasing delicacy of adjustments 

 of the various species to their whole environment. Thus 

 has arisen that multiplicity of species intermingled in the 

 same areas, none being able, as in the temperate zone, to 

 secure such a superior position as to monopolise large areas 

 to the exclusion of others. Hence also it has come about 

 that the equatorial species seem to be better defined more 

 sharply distinguished from each other than many of those 

 of the temperate and northern zones. They are what 

 Dr. Beccari terms first-grade species, as in the case of 

 Borneo, an island which forms part of what has quite 

 recently been an equatorial continental mass. It is interest- 

 ing to note that Mr. Th. D. A. Cockerell has arrived at a 

 similar conclusion from his study of the rich fossil flora of 



