12 INTRODUCTION 
(x) 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 
