452 INTRODUCTORY LECTURES. 



the same contagion, that is, from the same seed, all the es- 

 sential circumstances of the disease are produced; thus a 

 confluent smallpox may arise from a contagion of the mildest 

 kind, and vice versa. We believe, therefore, that the seed is 

 but one, and that the changes, the varieties, are analogous to 

 those produced in plants by culture, that is, that they depend 

 on the state of the body into which the contagion is conveyed. 

 This is the solid foundation of inoculation, that we have now 

 learned to modify the body in such a manner that the contagion, 

 when applied, will not give rise to those varieties and anomalies. 

 I shall add here what I think a curious corollary, viz. that the 

 specific nature of contagions, and the dependance of the vari- 

 ety of the disease upon the nature of the body, are presump- 

 tions in favour of the inoculation of all specific contagions. 

 When we shall have acquired some more experience with regard 

 to the manner of fitting the body, and of conducting the in- 

 oculation in other diseases, as well as we now do in the small- 

 pox, I am persuaded that the practice will be equally applicable. 

 I must own, however, that this manner of ascertaining the 

 species of diseases, by the propagation of a contagion, does not 

 go a great length, as there are not many diseases which we can 

 limit in this manner. But I obviate the whole objection in the 

 same manner as in the case of fossils, by saying, that there is 

 use for prosecuting the study of the varieties of diseases ; and 

 it is not of very great consequence to ascertain the limits be- 

 tween species and varieties ; for some steady conditions and cir- 

 cumstances, which accompany variety, require different practice, 

 and therefore are equally to be studied as what may be more 

 strictly called species. 



I have to add a curious particular with regard to this matter. 

 Diseases are distinguished by a single symptom, or by a con- 

 course of symptoms, and more frequently by the latter. But 

 wherever we find a concourse of symptoms constantly and very 

 frequently occurring together, and where this fixed concourse 

 does upon occasion occur, with a variety of other symptoms 

 added to it these additional symptoms, however, being some- 

 times absent and sometimes present I maintain that it is the first 

 concourse only which establishes the character of the disease. 

 There may be some use in taking notice of the occasional varieties, 



