nse
of smell in flies and cockroaches is connected with the antennae has
been shown by cutting them off: whereupon the insects can no longer find
carrion. In his work on _Earthworms_, Darwin shows that, though
sensitive to mechanical tremors, they are deaf (or, at least, not
sensitive to sonorous vibrations transmitted through the air), by the
following experiment. He placed a pot containing a worm that had come to
the surface, as usual at night, upon a table, whilst close by a piano
was violently played; but the worm took no notice of the noise. He then
placed the pot upon the piano, whilst it was being played, when the
worm, probably feeling mechanical vibrations, hastily slid back into its
burrow.

When, instead of altering one circumstance in an instance (which we have
done our best not otherwise to disturb) and then watching what follows,
we try to find two ready-made instances of a phenomenon, which only
differ in one other circumstance, it is, of course, still more difficult
to be sure that there is only one other circumstance in which they
differ. It may be worth while, however, to look for such instances.
Thus, that the temperature of ocean currents influences the climate of
the shores they wash, seems to be shown by the fact that the average
temperature of Newfoundland is lower than that of the Norwegian coast
some 15 deg. farther north. Both regions have great continents at their
back; and as the mountains of Norway are higher and capped with
perennial snow, we might expect a colder climate there: but the shore of
Norway is visited by the Gulf Stream, whilst the shore of Newfoundland
is traversed by a cold current from Greenland. Again, when in 1841 the
railway from Rouen to Paris was being built, gangs of English and gangs
of French workmen were employed upon it, and the English got through
about one-third more work per man than the French. It was suspected that
this difference was due to one other difference, namely, that the
English fed better, preferring beef to thin soup. Now, logically, it
might have been objected that the evidence was unsatisfactory, seeing
that the men differed in other things besides diet--in 'race' (say),
which explains so much and so easily. But the Frenchmen, having been
induced to try the same diet as the English, were, in a few days, able
to do as much work: so that the "two instances" were better than they
looked. It often happens that evidence, though logically questionable,