î.
, f
their normal habitation is in the deeper abysses of the ocean, whilst presumptive
evidence of the vitality of the Foraminifera with which they are associated is
afforded by the remains of these having been detected by me in the digestive
cavities of the radiate creatures in question.
Assuming these facts then for the present, we have to modify to a considerable
extent the usually received views regarding distribution and dm-ation of species,
to which attention has already been drawn.
Since we know that similar climates and conditions of the earth’s surface do
not necessarily involve the presence of the same kinds of organized beings, and,
on the other hand, that the association of similar forms does not involve the
existence of a similar climate, we ai'e justified in supposing that animals of the
same species (that is to say, originating from one specific progenitor), now
living under widely different conditions as to climate, have graduaUy become
habituated to the transition from one set of conditions to the other, during the
lapse of extended periods in which alterations in the distribution of sea and land,
of sufficient magnitude to have produced these climatal differences, have taken
place.
But it appears to me that this inference is warranted, whether we accept the
doctrine of single specific centres or that of the transmutation of species, since
it is conformable with all that is known of the duration of species to believe that
the period requisite to effect marked modifications in their characters is brief, in
comparison with that which is included between their first appearance on earth
and their final disappearance from it, and, hence, that the duration of any given
form may be regarded as often extending beyond the period occupied in any
single change in the distribution of land and sea. For it is evidently unphiloso-
phical to suppose that a species, once fairly established, should disappear from
the face of the earth before it had passed through that specific life which is as
essentially a law of nature as the life of the individual—and tantamount to an
assertion that creation is without a purpose, and under the influence of accidental
agencies.
Whatever views may be entertained regarding the origin of species, it is
universaUy admitted that they are associated together in distinct groups, each of
which is typical of the zoological province in which it occurs, in the same
manner that certain species are typical of each group'—the same group and the
same species never being repeated either in space or time, and the same species
never appearing in two or more provinces, except as an emigrant from its genetic
centre. By what laws the adaptation of particular regions for particular species
is determined we have at present no means of ascertaining, inasmuch as the
existence of similar cUmates neither involves the presence of similar species, nor
is the presence of similar species necessarily associated with the existence of
similar climates.
But whilst the distribution of living animals and plants has already been
defined throughout the entire terrestrial portion of the globe with a close
approach to accuracy, our knowledge of the distribution of the inhabitants of the
ocean has been almost entfrely restricted to coast-lines, and even as regards these
we have no systematic information beyond European seas. 'Fhe great zoological
provinces of the open ocean may therefore be regarded as unexplored, save in the
case of a limited number of free-floating organisms; and, as a natural consequence
of the antibiotic view respecting its deeper abysses, the entii'e submarine
provinces of life, vast and varied as they must be if life be there present at all,
remain as yet a terra incognita.
In our endeavours to trace the distribution of the lower microscopic forms of
animal and vegetable life, the difiiculties are still further enhanced owing to the
greatly increased liahihty to diffusion by accidental causes, and a large proportion
of the entire series having thus become common to every portion of the
globe. Nor is this world-wide dispersion confined to recent species; for, as
shown by Ehrenberg, it extends to many fossil forms. Amongst the Foraminifera,
liring species still abound in our seas which flouiished in the ancient seas of
the Mesozoic period, long before any of the now living species of the higher
orders of the animal kingdom had as yet made their appearance on earth. The
same universality of distribution is to be observed amongst the Polycystina, and,
on the vegetable side, amongst the Diatomacese and Desmideace®, although in
the fossil condition the two former have not hitherto been detected so far back
in time, and the latter are altogether unknown*. When the extremely minute
* I t has been customary to regard certain minute spinous bodies in the Cretaceous flints as
sporangial cells of Desmidiacece. The Rev. J. B. Reade was the first to draw attention to these bodies,
in a paper published in ‘ The Annals of Natural History ’ for 1838. Subsequently they were detected
by Mr. Deane in the substance of the Chalk itself, and were shrewdly surmised by him to be of animal
origin (Trans. Microseop. Soe. 1849, vol. ii.). In 1851, and again in 1857, the organisms referred to