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OF THE DOCTRINE OF VITAL AFFINITY. 395 
Liesic himself, that the infusory animals decompose the carbonic acid of the air, 
and exhale oxygen in like manner as vegetables; and the evidence of the forma- 
tion of oily out of saccharine or amylaceous matters in many animals appears to 
be unequivocal. The two distinct powers, therefore, of forming and of fixing, or 
appropriating the organic compounds, are not so accurately divided between the 
vegetable and animal world as has been thought. But the more that any physio- 
logist is convinced, as Dr Davseny is, that the formation of organic compounds is 
peculiar to vegetables, certainly the less reason can he have for supposing that 
this great change can be due to any mechanical movements, on the principle of 
contraction and impulse, arising out of the vital principle; the provisions for such 
movements being so striking a part of the economy of animals, and never having 
been proved to exist at all in vegetables. 
But, secondly, In maintaining the scientific correctness of the doctrine of 
vital affinity, as I have defined it, I think it quite unnecessary to go into these 
details. I maintain that the objections made to this doctrine, both by Dav- 
BENY and Humsoupt, are logically incorrect, because, in dealing with a set of 
facts so extraordinary, so important and characteristic as the chemical changes 
of living beings have been shewn-to be, they hold it to be incumbent on us 
to prove the negative proposition, that these may not ultimately be referred to 
those laws which regulate the chemical changes in dead matter, which may be 
acting under conditions not yet known, and of which they say nothing. The 
rule of sound logic is,—‘“ afirmantibus incumbit probatio.” It is admitted on 
all hands, that the phenomena of life in general are so peculiar and important 
as to be properly ranked together as a separate science; and we have shewn that 
of these phenomena, the most essential and characteristic are certain chemical 
changes, which are admitted to be so distinct from any that can be observed any- 
where else in nature as to “indicate the existence of a power distinct from any 
simply chemical or physical forces.” It is clearly incumbent on those who main- 
_ tain that, nevertheless, these ordinary chemical forces, acting under conditions 
not yet understood, may be found adequate to this explanation, to give evidence 
in the way of observation and experiment of this proposition, otherwise their 
doctrine is only a hypothesis. If the subject is not thought worthy of scientific 
inquiry at all, then Physiology is not a separate science. If it is regarded as a 
separate science, of equal interest and importance as any other, then it is the 
duty of physiologists, acting on the strict method of induction,—because ascend- 
_ ing from facts to principles, instead of descending from principles to facts,—to 
examine these individual phenomena themselves, arrange and classify them as 
they present themselves in the different classes of living beings, and consider how 
far laws deduced from the observation of dead matter can go in the explanation of 
them; but wherever we find that there is a difficulty in that explanation,—in- 
