30 THE WORLD OF LIFE 



life and are also most varied in soil, contour, water-supply, 

 etc., will, when in a state of nature, be occupied by a large 

 proportion of the common and widespread plants, together 

 with so many of the less common or the rare species which 

 find the requisite conditions in some part of its varied soil 

 and aspects, as to produce that crowding together of species 

 and luxuriance of growth which are such a joy to the botanist 

 as well as to the less instructed lover of nature. 



All these peculiarities of vegetation are to be met with in 

 every part of the world, and often in a more marked degree 

 than with us. But this depends very much on diversities of 

 climate and on the extent of land surface on which the entire 

 flora has been developed. The total number of species de- 

 pends mainly on these two factors, and especially on the 

 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 mois- 

 ture 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 extend- 

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

 Himalavas in north-eastern India, where the monsoon winds 

 carrv 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 



