[v] 



AUTHOR'S PREFACE 



The delimitation of separate floral districts and their grouping 

 into more comprehensive combinations are nearly completed, and 

 the time is not far distant when all species of plants and their 

 geographical distribution will be well known. The objects of 

 geographical botany will not, however, then be attained, as is 

 often assumed, but a foundation merely will have been laid on 

 which science can construct a larger edifice. The essential aim 

 of geographical botany will then be an inquiry into the causes of 

 differences existing among the various floras. 



Existing floras exhibit only one moment in the history of the 

 earth's vegetation. A transformation which is sometimes rapid, 

 sometimes slow, but always continuous, is wrought by the reciprocal 

 action of the innate variability of plants and of the variability of 

 the external factors. This change is due partly to the migrations 

 of plants, but chiefly to a transformation of the plants covering the 

 earth. Owing to unknown internal causes, the structure of plants 

 is subject to a process of metamorphosis, which taken as a whole 

 is slow, but apparently uninterrupted, and which gives rise to purely 

 morphological differentiation, i. e. to the acquisition of characters 

 bearing no apparent relation to the environment. Experience shows, 

 however, that this differentiation is profoundly and rapidly modified 

 by changes in the environment, every one of which immediately 

 involves a change in the organization of the plants. If the new 

 characters be useful, they are selected and perfected in the de- 

 scendants, and constitute the so-called ' adaptations ' in which the 

 external factors acting on the plants are reflected. Since these 

 last change with the geographical position, it is by the adaptations 

 that the causes of the differences in the facies of the vegetation at 

 different points on the earth are rendered more comprehensible, so 

 that their investigation is to be numbered among the chief duties 

 of geographical botany. 



