BENTHAM ON THE HISTORY AND GENERA OF PLANTS. 135 



many American, African, and Asiatic species in large masses of speci- 

 mens. In some of the more important monographs I have worked up, I 

 have been enabled to compare the materials of the principal herbaria of 

 Europe ; and, since my working- stock has been transferred to Kew, the 

 daily consultation of such a collection as that of Sir "William Hooker has 

 contributed very much to confirm my ideas as to the variability and 

 limitation of species ; and nothing more so than the extensive and highly 

 instructive series brought from India by Drs. Hooker and Thomson, and 

 the numerous accurate and judicious notes and memoranda so liberally 

 communicated by Dr. Hooker. "When, therefore, I speak of having ob- 

 served a series of specimens collected in various parts of the geographi- 

 cal area of a species, I do not mean (as has been hinted) the examination 

 of a few single specimens from different localities deposited in a herba- 

 rium, but the observation of a species in a living wild state in diffe- 

 rent countries, or the comparison of numerous specimens, either pro- 

 miscuously collected, or selected, with notes, for the purpose of illus- 

 trating variations. 



And here I would observe, that the use of herbaria in determining 

 the extent of variability of species requires the greatest caution. JSTot 

 only are the specimens preserved generally unaccompanied by any notes 

 on the comparative frequency of the form gathered, and others closely 

 resembling it, or on any other local circumstances affecting the ques- 

 tion, but they are very likely to lead the botanist astray in these parti- 

 culars. A collector is naturally struck by a plant differing in aspect 

 from the generality of its species, and gathers it in preference to the 

 forms more familiar to him. The consequence is, that it frequently hap- 

 pens that an accidentally abnormal variety, which may occur only once 

 in the way in nature, having been cut up into a number of specimens, 

 and distributed without notes to various botanists, has, from its presence 

 in so many herbaria, all the appearance of a form abundant in the loca- 

 lity cited. 



The experience I have thus obtained has gradually produced in my 

 mind a conviction of the truth of the following axioms : — 



Every species has certain determinate limits of variation, which it 

 only exceeds under exceptional circumstances. 



The exceptionally abnormal forms thus produced are few in indivi- 

 duals, and are not reproduced ; or their race becomes gradually extin- 

 guished, when the causes which produce them cease. 



Within these limits of variation, a species will, in some countries, or 

 under certain circumstances, produce an indefinite number of indivi- 

 dual, or more or less permanent varieties, "often passing into each other 

 by almost imperceptible gradations ; whilst, in other countries, or un- 

 der other circumstances, a certain number of these varieties or races will 

 remain, generation after generation, marked by positive, distinctive 

 characters, having at first sight the appearance of real species. 



Plants of the same species often breed freely together, the cross- 

 breeding of different individuals sometimes producing a more vigorous 

 offspring than those sprung from a single flower, and being, perhaps, oc- 



