752 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



For vegetation is always developing; floras occupy only a moment of vegetational 

 history; and it is the relation of structure, function, and environment which 

 must be studied. 



It was very early recognized that plant geography is a matter which 

 involves, in a peculiarly direct way, the fundamental conceptions of both space 

 and time, and that the subject might, therefore, be approached from one or 

 other of these directions or from some combination of both. Thus there has 

 always been in plant geography an underlying triplicity; the swing of emphasis 

 within this provides the background to the history of the subject. Just how the 

 three streams or branches of plant geography should be defined and named has 

 been a matter of considerable argument. Those who are interested in terminolo- 

 gies will find excellent accounts of this by Riibel (1927)' and by Wulff (1943), 

 but it is more convenient here to describe this triplicity in rather freer terms. 

 First, there is the stream in which the main emphasis is the correlation of space 

 and form; this has been the special concern of those plant geographers, like 

 de Candolle, Hooker, Asa Gray, and Engler, who have also been preeminent 

 systcmatists. It may be called the taxonomic stream. Second, there is the stream 

 in which the strongest emphasis is placed upon the historical and developmental 

 aspects of the subject, and this is perhaps most often now called the historical 

 stream. Third, there is the stream in which the two conceptions of space and 

 time are balanced more evenly than in either of the others, namely ecology, which 

 is mainly concerned with the distributional changes, usually by their nature 

 relatively small, resulting from the gradual changes in a mutable environment. 



The main *point in relation to this analysis is very clear : the coming of Dar- 

 winism shifted the emphasis away from the first stream, where it had in fact 

 been almost wholly concentrated, and distributed it more evenly among all three. 

 For a good many years the full effect of this was not felt because this was a 

 period of reorientation, but once this adjustment had been made, the rapid 

 development of the streams which had been so long held back by the pre-evolu- 

 tionary conception of the cosmas was inevitable. For reasons which we need not 

 attempt to specify too closelj^ but which are certainly connected with the full 

 flowering of the idea of adaptation to environment the ecological stream was the 

 first to break through. 



So simple an analysis is likely to be too clear-cut to depict the whole truth, 

 and this is certainly so here, especially with regard to the first two streams, 

 between which there has always been a close connection. We may indeed recog- 

 nize two streams, but there is, as it were, a constant interchange of water between 

 them. The ecological stream, however, has much more noticeably scoured its own 

 channel and, although this stream flows alongside the others, there is little actual 

 communication between them. The reasons for this would make a most interesting 

 study, for they are probably not all purely botanical, but this is no place to 

 attempt it. We must content ourselves with the statement that what is now 

 called plant ecology became in a comparatively short time largely divorced from 

 the other aspects of plant geography. There is therefore both reason and excuse 

 for referring only briefly to it here, apart from the fact that, since ecology was 

 unknoAvn as a separate study in 1853, it may formally be considered outside the 

 terms of present reference. 



It is difficult to mark the exact point at which plant ecology became estab- 



