the presence of which it has not appeared
may be set aside as fortuitous, as being not an indispensable
antecedent. This is really the guiding principle of the method as a
method of observation.

Let the inquiry, for example, be into the cause of Endemic Goitre.
Instances of the disease have been collected from the medical
observations of all countries over many years. Why is it endemic in
some localities and not in others? We proceed on the assumption that
the cause, whatever it is, must be some circumstance common to all
localities where it is endemic. If any such circumstance is obvious
at once, we may conclude on the mere principle of repeated coincidence
that there is causal connexion between it and the disease, and
continue our inquiry into the nature of the connexion. But if no such
circumstance is obvious, then in the course of our search for it we
eliminate, as fortuitous, conditions that are present in some cases
but absent in others. One of the earliest theories was that endemic
goitre was connected with the altitude and configuration of the
ground, some notorious centres of it being deeply cleft mountain
valleys, with little air and wind and damp marshy soil. But wider
observation found it in many valleys neither narrower nor deeper than
others that were exempt, and also in wide exposed valleys such as
the Aar. Was it due to the geological formation? This also had to
be abandoned, for the disease is often incident within very narrow
limits, occurring in some villages and sparing others though the
geological formation is absolutely the same. Was it due to the
character of the drinking-water? Especially to the presence of lime
or magnesia? This theory was held strongly, and certain springs
characterised as goitre-springs. But the springs in some goitre
centres show not a trace of magnesia. The comparative immunity of
coast regions suggested that it might be owing to a deficiency of
iodine in the drinking-water and the air, and many instances were
adduced in favour of this. But further inquiries made out the presence
of iodine in considerable quantities, in the air, the water, and the
vegetation of districts where goitre was widely prevalent; while in
Cuba it is said that not a trace of iodine is discoverable either in
the air or the water, and yet it is quite free from goitre. After
a huge multiplication of instances, resulting in the elimination of
every local condition that had been suggested as a possible