209 
of Edinburgh ^ Session 1863-64. 
Entertaining these views of the services which my distinguished 
friend has performed to Science and the Arts, I have much pleasure 
in expressing the same sentiments on the part of the Society, and 
in being their organ in delivering to him the Prize which he has so 
justly merited. 
In discharging this duty, I am proud to think, and I am sure 
that all here will participate in the sentiment, that Scottish Science 
has such a representative in the University of the West, while, in 
our own, it has one of kindred genius and power. 
Professor Thomson, I now beg to present to you the Keith Medal, 
and to congratulate you on the honour which you have so justly 
deserved. 
1. On Vital Agency with reference to the Correlation of 
Forces. By William Seller, M.D,, F.E.S.E., Fellow of the 
Koyal College of Physicians of Edinburgh. 
In this paper there is presented a series of characteristic examples 
of vital agency, followed by some considerations .bearing on the 
extent to which, in a physiological point of view, it can be ad- 
mitted that the physical forces, under exchangeable forms, are 
concerned in organic phenomena. At the same time, one of the 
particular objects of inquiry is, whether there be ground for the 
belief that vital agency, being not a force but a directive principle, 
may accomplish all the parts assigned to it, solely by means of 
the physical forces variously modified through organic structure. 
It is laid down among the preliminaries, that no facts hitherto 
discovered bring the origin of organic life within the pale of in- 
ductive inquiry ; that the transmutation of species by natural 
selection, though a brilliant conception, holds as yet no foundation 
of an inductive character ; that it remains therefore a fundamental 
fact in physiology, admitting of no explanation, that all species 
have their commencement in original parentage ; and further, that 
it is thence correct in principle to maintain that all the multiplied 
phenomena in the life of a species, of whatever extent, derive their 
source from the potentiality of the reproductive cells in the pri- 
mordial parent or parents of that species. 
It is also taken for granted, that such a directive principle, as 
