the Pentland Hills, near Edinburgh, into that city,
which Dr Johnson regarded as by no means abundantly supplied with the
"pure element of water," it was necessary to compensate the mill-owners
by another supply. Accordingly a valley, (the supposed scene of Allan
Ramsay's "Gentle Shepherd,") through which there flowed a small stream,
had a great embankment thrown across it. After this operation, of course
the waters of the upper portion of the stream speedily rose to a level
with the sluices, thus forming a small lake, commonly called the
"Compensation Pond." The flow of water now escapes by throwing itself
over the outer side of the embankment, which is lofty and precipitous,
in the form of a cataract, up which no fish can possibly ascend. Yet in
the pond itself we have recently ascertained the existence of sea-trout
in a healthy state, although such as we have examined, being young, were
of small size. These attributes, however, were all the more important as
proving the breeding condition of the parents in a state of prolonged
captivity. It is obvious that sea-trout must have made their way (in
fulfilment of their natural migratory instinct) into the higher portions
of the stream prior to the completion of the obstructing dam; and as
none could have ascended since, it follows that the individuals in
question (themselves and their descendants) must have lived and bred in
fresh water, without access to the sea, for a continuous period of
nearly twenty years. This is not only a curious fact in the natural
history of the species, but it is one of some importance in an
economical point of view. Sea-trout, as an article of diet, are much
more valuable than river-trout; and if it can be ascertained that they
breed freely, and live healthily, without the necessity of access to the
sea, it would then become the duty, as it would doubtless be the desire,
of those engaged in the construction of artificial ponds, to stock those
receptacles rather with the former than the latter.[25]

[24] Mr Shaw informs us, moreover, that if those individuals
which have assumed the silvery lustre be forcibly detained for
a month or two in fresh water, they will resume the coloured
coating which they formerly bore. The captive females, he adds,
manifested symptoms of being in a breeding state by the
beginning of the autumn of their third year. They were, in
truth, at this time as old as _herlings_, though not of