8 INTRODUCTION. 



is so great (probably, according to the estimate of some 

 writers, 200,000), that to enable us to survey this mass, a 

 scientific expedient has been requisite, in the shape of a 

 systematic arrangement of the different genera. Fortu- 

 nately, Nature meets us half-way. In all the external 

 forms, in the number, the arrangement and the structure 

 of the separate parts, in the laws which regulate their 

 development, the larger groups of species of plants exhibit 

 a great agreement among themselves, and by this very 

 fact are distinguished from other groups. Who can 

 notice, at the time of blossoming, a carrot, the hemlock, 

 parsley, chervil, anise, dill and the rest, without being struck 

 with the agreement of these plants in general structure ? 

 Who would not, in like manner, perceive the close relation 

 which exists between the various kinds of cabbage, the 

 mustard, horse-radish, radish, turnip and similar plants ? 

 Any one who examines a little more closely, will recognize 

 a multitude of plants which are distinguished by a strong 

 aroma, the balm, mint, sage, thyme, marjoram, lavender 

 &c., and a wonderful agreement of structure will be 

 observed. Thus Nature herself indicates to us the path we 

 are to take ; following out such traces, Botanists have 

 gradually recognized and characterized a great number of 

 these groups of plants, which they have called Families. 

 That in this, as in the foregoing case, there is no space 

 here for completeness, need scarcely be mentioned, but we 

 could not deny it to ourselves to take one family, as an 

 example, and characterize it more accurately (ix). 



In the group selected, the Cactus plants, our attention is 

 attracted, among many other things, to their remarkable 

 distribution over a comparatively small part of the earth's 

 surface ; and this leads very naturally to the question, how 

 the different species of plants spread themselves over the 



