re often legitimately comes first. It is the forecast of
genius which anticipates the fact and constitutes a spur towards its
discovery. If, instead of being a spur, the theoretic guess rendered
men content with imperfect knowledge, it would be a thing to be
deprecated. But in modern investigation this is distinctly not the
case; Darwin's theory, for example, like the undulatory theory, has
been a motive power and not an anodyne. 'At last,' continues
Professor Virchow, 'in the nineteenth century we have begun little by
little really to find _contagia animata_.' So much the more honour, I
infer, is due to those who, three centuries in advance, so put
together the facts and analogies of contagious disease as to divine
its root and character. Professor Virchow seems to deprecate the
'obstinacy' with which this notion of a _contagium vivum_ emerged. Here
I should not be inclined to follow him; because I do not know, nor
does he tell me, how much the discovery of facts in the nineteenth
century is indebted to the stimulus derived from the theoretic
discussions of preceding centuries. The genesis of scientific ideas
is a subject of profound interest and importance. He would be but a
poor philosopher who would sever modern chemistry from the efforts of
the alchemists, who would detach modern atomic doctrines from the
speculations of Lucretius and his predecessors, or who would claim for
our present knowledge of _contagia_ an origin altogether independent of
the efforts of our 'forefathers' to penetrate this enigma.

*****

Finally, I do not know that I should agree with Professor Virchow as
to what a theory is or ought to be. I call a theory a principle or
conception of the mind which accounts for observed facts, and which
helps us to look for and predict facts not yet observed. Every new
discovery which fits into a theory strengthens it. The theory is not
a thing complete from the first, but a thing which grows, as it were
asymptotically, towards certainty. Darwin's theory, as pointed out
nine and ten years ago by Helmholtz and Hooker, was then exactly in
this condition of growth; and had they to speak of the subject to-day
they would be able to announce an enormous strengthening of the
theoretic fibre. Fissures in continuity which then existed, and which
left little hope of being ever spanned, have been since filled in, so
that the further the theory is tested the more fully does it harmonise
with progressive ex