12 INTRODUCTION 



(i) the modes and (2) the causes by which these things 

 have come to pass.' 



The differences between existing species are open 

 to study in more than one way. By way of limiting 

 the discussion for the present, we shall consider the 

 case of plant species only ; but the methods of study 

 which are applicable to animal species are of quite 

 similar kinds. 



Four methods at least are available. Firstly, that 

 of comparison ; secondly, the method of statistical 

 examination ; thirdly, the method of cultural experi- 

 ment ; and lastly, that of cross-breeding. 



The method of comparison is the one to which the 

 ordinary worker in descriptive botany is almost of 

 necessity confined. In this way plants which closely 

 resemble one another are grouped together as belong- 

 ing to the same species, and separated from others, 

 the appearance of which is different. By appearance 

 is not meant simply the general habit of the plants ; 

 all morphological features whatever may be used for 

 purposes of comparison, and the most minute are 

 often of the greatest importance. But the systema- 

 tist who works only in this way knows nothing of the 

 real relationships between the plants with which he 

 is dealing. 



When a sufficient number of specimens is available, 

 the methods of statistics can be applied. These 

 involve the making of a series of accurate measure- 

 ments or countings of the parts upon which depend 

 the supposed differences or resemblances of the plants 



