ON NOSOLOGY. 451 



tanists found that, on many occasions, they could trace the pro- 

 duction of the variety to a different culture ; but the matter 

 was not so easily determined with respect to plants found in their 

 natural places, where the variety was not the effect of culture 

 or soil ; and here they had recourse to a much more effectual 

 means, which was by observing, whatever was the variety, if the 

 seed taken from one plant brought up a plant of the same kind, 

 and without these varieties ; all such as they found to be thus 

 steadily propagated, they considered as varieties of the same 

 species only ; but what did not follow the seed, they considered 

 as different species : and by this means the species of plants, 

 which amounted to twenty thousand, have now been reduced to 

 nearly half that number. The same difficulties occur in Zo- 

 ology, and the same means, viz. the propagation in the way of 

 generation, applies there also with advantage, but not as yet so 

 perfectly and with such ease as in botany. In the third king- 

 dom, Mineralogy, we have no such means of distinguishing be- 

 tween species and varieties, and therefore the matter is left in 

 much uncertainty. We can in mineralogy go a certain length 

 in establishing distinctions of genera, and, in many cases, those 

 of species ; but beyond these there is a very considerable num- 

 ber of varieties which we still need to enumerate. The taking 

 notice of the varieties in fossils, and prosecuting them, is of 

 much more use than it is with respect to plants ; for in plants 

 a change of virtue does not follow a variety ; but in fossils the 

 quality of the ore, and the quantity of the metal, are often con- 

 nected with certain varieties ; and, therefore, we allow the study 

 of varieties to remain here, because it is in a great measure of 

 use. 



Now, with regard to Nosology, which is our province, we can 

 go somewhat farther than in Mineralogy ; for we can there 

 find something analogous to the propagation of seed in living 

 bodies. We observe this in the case of all contagions, particu- 

 larly in those we call specific contagions ; and, as far as my ob- 

 servation goes, even in those which are not strictly speci- 

 fic, when we can trace a disease to its contagion, we can, 

 in some measure, fix its species. Thus, in the case of small- 

 pox, a great many varieties have been marked, but they are 

 varieties only of one species : a proof of which is, that from 



