544 



DR BARCLAY 



pair nor carry on its destined operations, it deserts 

 them entirely, leaving them to the fate of their 

 own inherent chemical affinities, which sooner or 

 later dissolve the temporary arrangements of their 

 particles, and mingle them again with the mass of 

 dust from which they had been taken. 



If the philosopher be anxious to know what is 

 the nature of these Agents that thus organise the 

 various ^structures of plants and animals, he can 

 only know them from their Effects ; and as these 

 are different in different structures, he may safely 

 conclude, that their orders, genera, and species, 

 are as many as are the orders, genera and species 

 of the structures which they form. Of their real 

 essence^ as of that of any other substance in na- 

 ture, he must be content to remain ignorant. But 

 should he wish merely to know them, as he knows 

 other substances, from their effects^ in that way 

 he may know more of them than of any others 

 that have ever happened to engage his attention. 

 No substances produce such a number or such a 

 variety of specific effects, or in such a regular or- 

 der of succession as they do in constructing their 

 systems ; and, in acting through the medium of 

 these systems, no substances exhibit such a num- 

 ber or such a diversity of specific operations ; 

 while the systems themselves, in each species, dis- 

 play additional discriminating characters, in their 

 form, magnitude, and colours ; in the number of 

 their organs, in the varieties of their kinds, in 



