250 A LIFE'S WORK IN IRELAND. 



mines the mode in which temperature is partitioned 

 throughout the twenty-four hours of the day is the 

 amount of cloud and the degree of moisture in the 

 atmosphere ; for a knowledge of which we must look 

 to the rainfall through the months of the year as 

 furnishing tlie best available key. 



" The rainfall affects plants directly through the 

 nourishment it conveys to them, and indirectly 

 through the state of the sky which its amount or 

 absence implies. Indeed, so great is the influence 

 of rainfall on vegetation, that we cannot be far ^vrong 

 in regarding it as co-ordinate with that of tempera- 

 ture. Whatever the law may be which expresses 

 the atmospheric conditions that determine the limits 

 of the growth of species, it must include in its 

 functions both the heat and moisture of the air. 



" Decandolle deduced the law for the distribution 

 of species over a region whose climates are marked 

 off from each other rather by variations of temper- 

 ature than of moisture. He then endeavoured to 

 extend it so as to account for the distribution of 

 the florae of other regions, the climates of which 

 may be characterised either as moist at all seasons 

 or subject to marked variations of moisture at stated 

 seasons. Perhaps not the least valuable of the re- 

 sults arrived at by him is the negative one stated in 

 these words : — ' On the borders of the Mediterranean 

 Sea, the limits appeared so often determined by the 

 humidity, or by causes still unknown, that the 



