DISTKIEUTION AND EVOLUTION 103 



No less indicative of delicate response to variation of tem- 

 perature, and therefore of close adaptation to the whole modi- 

 fied environment, is the continuous increase in the number 

 of species with every important change of latitude. Although 

 this increase is but slight for moderate changes, and is there- 

 fore liable to be masked by other favourable or adverse con- 

 ditions 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 pronounced 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 temper- 

 ate 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 neces- 

 sarily 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 importance to vege- 

 tation so long as other conditions remain tolerably uniform 

 and favourable. 



It is this long-continued uniformity of favourable condi- 

 tions within the tropics, or more properly within the gi'eat 

 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 monoj)olise large areas to the exclu- 



