28 THE WORLD OF LIFE 



former. The variety of species is small in arctic or sub-arctic 

 lands, where the long and severe winter allows of only certain 

 forms of vegetable and animal life ; and it is equally if not 

 more limited in those desert regions caused by the scarcity 

 or almost complete absence of streams and of rain. It is 

 most luxuriant and most varied in that portion of the tropics 

 where the temperature is high and uniform and the supply of 

 moisture large and constant, conditions which are found at 

 their maximum in the Equatorial Zone within twelve or fifteen 

 degrees on each side of the equator, but sometimes extending 

 to beyond the northern tropic, as on the flanks of the 

 Himalayas in north-eastern India, where the monsoon winds 

 carry so much moisture from the heated Indian Ocean as to 

 produce forests of tropical luxuriance in latitudes where most 

 other parts of the world are more or less arid, and very often 

 absolute deserts. 



Temperate Floras compared 



I will now endeavour to compare some of the chief floras 

 of the Temperate Zone, both as regards the total number of 

 species in fairly comparable areas, and the slight but clearly 

 marked increase of the number in more southern as compared 

 with more northern latitudes. 



I will first show how this law applies even in the com- 

 paratively slight difference of latitude and climate within our 

 own country. Dividing Great Britain (without Wales) 1 into 

 three nearly equal portions — Scotland north of the Forth and 

 Clyde, Mid -Britain, and South Britain, including all the 

 southern counties; with areas of 22,000, 26,000, and 31,000 

 square miles — the number of species (in 1 8 70) was, respectively, 

 930, 1 148, and 1230. At the same period the total of 

 Great Britain was 1425 species. These figures are all 

 obtained from Mr. H. C. Watson's Cybele Britannica, and 

 must therefore be considered to be fairly comparable. We 

 see here that the whole of the Scottish Highlands, with their 

 rich alpine and sub-alpine flora, together with that of the 

 sheltered valleys, lakes, and mountainous islands of the 

 west coast, is yet decidedly less rich in species than Mid- 



1 Wales is omitted in order to make the three divisions more equal, and con- 

 trasted in latitude only. 



