lf find it again. But the dog runs forward, notes vaguely by ear
and by eye the spot where it strikes, and then commences a systematic
circling within about ten yards of the spot. In half a minute he
pounces with the utmost assurance on to one selected stone, and brings
it to me. It is invariably the stone which had been in my hand, unseen
by the dog, thrown by me, and detected by the smell I have
communicated to it.

Not only is the discrimination of individuals by the sense of smell a
very astonishing thing, but so also is the obvious fact that the total
amount of odoriferous matter which is sufficient to give a definite
and discriminative sensation through the organ of smell is of a
minuteness beyond all calculation or conception. These two facts--the
almost infinite individual diversity of smell and the almost infinite
minuteness of the particles exciting it--render it very difficult to
form a satisfactory conclusion as to the nature of those particles. It
has been from time to time suggested that the end organs of the
olfactory nerves may be excited, not by chemically active particles,
but by "rays," olfactive undulations comparable to those of light.
Physicists have not yet been able to deal with the problem, but the
recent discoveries and theories as to radio-active bodies such as
radium may possibly lead to some more plausible theory as to the
diffusion and minuteness of odorous particles than any which has yet
been formulated. An example of the minuteness of odoriferous particles
is afforded by a piece of musk which for ten years in succession has
given off into the changing air of an ordinary room "particles"
causing a readily recognised smell of musk, and yet is found at the
end of that time to have lost no weight, that is to say, no weight
which can be appreciated by the finest chemical balance. An analogy (I
say only an analogy, a resemblance) to this is furnished by a pinch of
the salt known as radium chloride, no bigger than a rape-seed, and
enclosed in a glass tube, which will continue for months and years to
emit penetrating particles producing continuously without cessation
most obvious luminous and electrical effects upon distant objects, the
particles being so minute that no loss of weight can be detected in
the pinch of salt from which they are given off.

The sense of smell is of service to animals--

(1) In avoiding enemies and noxious things.

(2) In tracing and following and discriminating p