xxiv Proceedings of Royal Society of Edinburgh. 
Himself never an extensive traveller, he yet, by careful and 
systematic collection of facts, prepared himself to he the author in 
1855 of the Geographie hotanique raisonnee^ which is considered to 
be his most important work. It was not his object to compile from 
books of travel a description of the vegetation of the earth, nor 
did he attempt to explain all the known phenomena of distribution 
of plants. In his own words his object was “ to seek out the laws 
of the distribution of plants upon the earth, by means of a limited 
number of facts, which should serve as a basis, and proofs”; ’‘'‘rerum 
cognoscere caiisas should be the goal in all true science.” And 
again, the principal object should be to show in the distribution of 
plants as they are, what may be explained by the actual conditions 
of climate, and what depends upon anterior conditions. The work 
was divided into three parts : the first dealt with the mode of action 
of temperature, light, and moisture upon plants ; the second with 
plants from the point of view of their distribution on the globe, the 
causes of their origin, their frequency or rarity ; in the third the 
different countries were studied from the point of view of their 
vegetation. 
His introduction of a modified method of the sum of temperatures 
was perhaps the most important point. Boussingault had already 
introduced the method, calculating the sum of temperatures upon 
the rough thermometric mean. De Candolle showed that the true 
method of sums of temperatures consists in calculating them above 
a certain minimum, from which point the vital phenomena of the 
plant in question begin to be active. Each species extends further 
northwards as far as it finds a certain fixed sum of heat, thus 
calculated, between the day when a certain mean temperature 
commences, and that when it ends ; but these rough results are 
modified by other conditions ; still, though not mathematically 
exact, the method laid down by him gives useful results in con- 
nection with the study of the geographical limits of species. 
These and kindred subjects occupied the attention of de 
Candolle repeatedly in later years ; the most important of his 
later geographical writings being that in which he distinguished 
among plants six “ physiological groups.” In these were associated 
together plants which behave alike with regard to heat and 
moisture, and which accordingly may have together passed through 
