Perhaps the most effective aid to the investigation of natural
history which the present age has produced is the invention of the
aquarium, and particularly its application to marine forms of life.
Depending on that grand principle of organic chemistry, of world-wide
prevalence, that the emanations from animals and vegetables are
respectively essential to the continued life each of the other, it
was discovered that the relative proportions of number and bulk in
which organic beings of the two kinds could healthfully live together
was easily determined; and since the fact that the creatures were
inhabitants of water, whether fresh or salt, presented no exception to
the universality of the law, they had but to be placed together in a
suitable ratio, enclosed in a vessel containing water, and an aquarium
was established. Improvements in the form of the vessel, in the mode
of exposing the contents to observation, in the impact of the rays of
light, in the arrangement of the interior, and other points of value,
have indeed been progressively made; whereby the practical availability
of the invention for the purposes of experimental natural history has
been augmented; but some of us have found little difficulty, even
from the very first announcement of the discovery, in maintaining
the collections of sea-water, with their living plants and animals,
unchanged from year’s end to year’s end. I may be perhaps excused for
observing, that I have at present in use a large tank, full of marine
creatures, in which the water has been unchanged for four years, and
on which I look with peculiar interest, because it was the first tank
ever made for private use. This very aquarium has afforded, and still
affords the opportunity for the observation of many interesting details
of the structure and habits of the lower forms of animal life, details
which constitute the basis not only of my works on marine natural
history already published, but of the present series of papers also. We
collect the creatures, indeed, abroad, and there gather up some broad
facts of interest concerning their modes of life; but it is at home,
in the quiet of the study, with conveniences and aids to examination,
experiment, and record at command, that they must be studied. The
aquarium becomes in fact an apparatus, whereby we bring a portion of
the sea, with its rocks, and weeds, and creatures, to the side of our
study-table, and maintain it there.