EXAMINATION OR DISSECTION. 77 



he has thus been referred, he must revert to the beginning and carefully 

 go through every step of the investigation before he can be satisfied. 

 A fresh examination of his specimen, or of others of the same plant, a 

 critical consideration of the meaning of every expression of the cha- 

 racters given, may lead him to detect some minute point overlooked or 

 mistaken, and put him into the right way. Species vary within limits 

 which it is often very difficult to express in words, and it proves often 

 impossible in framing these analytical tables so to divide the genera 

 and species, that those which come under one alternative should 

 absolutely exclude the others. In such doubtful cases both alternatives 

 must be tried before the student can come to the conclusion that his 

 plant is not contained in the Flora, or that it is erroneously described. 



' In those Floras where analytical tables are not given, the student 

 is usually guided to the most important or prominent characters of each 

 genus or species, either by a general summary pi-efixed to the genera of 

 an order or to the species of the genus, for all such genera or species ; 

 or by a special summary immediately preceding the detailed descrip- 

 tion of each genus or species. In the latter case this summary is called 

 a diagnosis. Or sometimes the important characters are only indicated 

 by italicizing them in the detailed description. 



1 It may also happen that the specimen gathered may present some 

 occasional or accidental anomalies peculiar to that single one, or to a 

 very few individuals, which may prevent the species from being at once 

 recognised by its technical characters.' 



