NATURE OF THE EVIDENCE 15 



Selection that it was the sole agent in Evolution; 

 he regarded it as "the most important, but not 

 the exclusive means of modification" (Origin of 

 Species, p. 4). It is probable that other causes 

 have played a considerable part. 



I do not, however, propose to pursue the sub- 

 ject of the causes of Evolution. My purpose 

 has only been to show how Darwin, by discover- 

 ing in Natural Selection a true cause, though not 

 the sole cause of Evolution, at once made that 

 doctrine credible, and ultimately secured its 

 universal acceptance. 



The object of this book is to try to trace his- 

 torically the course which the evolution of plants 

 has actually followed a most difficult prob- 

 lem, but one on which the work of the last few 

 years has thrown light at several points. We 

 shall confine ourselves to those groups of plants 

 for which the evidence is the most satisfactory; 

 these groups are, generally speaking, the highest 

 and also the most familiar. 



The evidence on which we have to rely comes 

 partly from the comparative study of plants now 

 living. A vast amount of work has been done, 

 during the last two centuries, hi finding out the 

 relationships of plants and thus building up a 

 natural system of classification. In so far as 

 the attempt is successful, the result is to group 

 plants into families, of which the members, if 



