INTRODUCTION. 



WHOEVER has paid the slightest attention to the classifica- 

 tion of natural objects, whether plants or animals, must be 

 aware, that if we desire to follow natural principles in form- 

 ing our groups, that is, to bring together such species as 

 resemble each other in habit, properties and structure, it 

 is a vain task to attempt to define, with absolute strictness, 

 the classes into which we are forced to combine them. At 

 least, no effort to effect this desirable object has yet been 

 successful. Natural groups are so interwoven into each 

 other, and often exhibit such an exaltation and degradation 

 of characters within the limits of an Order or a Genus, 

 that the distinctive marks, as they approach each other, 

 gradually disappear, and two tribes, which in the more 

 highly developed species scarcely resemble each other, 

 are found, in the lower, to be either undistinguishable or 

 with difficulty distinguished. Thus, to a common observer, 

 the Poppy and the Fumitory would scarcely be supposed 

 to be closely related ; yet there is so much gradation be- 

 tween them, through allied genera, that some botanists 

 have placed them in the same Natural Order. Still more 

 unlike in appearance are the Rose and the Shamrock, yet 

 they belong to Orders so closely connected, that the only 

 invariable mark by which they can be distinguished is, 

 that in one, what is called the odd segment of the calyx 

 is posterior, while in the other it is anterior ; and till this 



