have only come into existence later; and, however large a proportion of
true cretaceous forms may be discovered in the deep sea, the modern types
associated with them must be abolished before the Fauna, as a whole,
could, with any propriety, be termed Cretaceous.


I have now indicated some of the chief lines of Biological inquiry, in
which the _Challenger_ has special opportunities for doing good service,
and in following which she will be carrying out the work already
commenced by the _Lightning_ and _Porcupine_ in their cruises of 1868 and
subsequent years.

But biology, in the long run, rests upon physics, and the first condition
for arriving at a sound theory of distribution in the deep sea, is the
precise ascertainment of the conditions of life; or, in other words, a
full knowledge of all those phenomena which are embraced under the head
of the Physical Geography of the Ocean.

Excellent work has already been done in this direction, chiefly under the
superintendence of Dr. Carpenter, by the _Lightning_ and the
_Porcupine_,[10] and some data of fundamental importance to the physical
geography of the sea have been fixed beyond a doubt.

[Footnote 10: _Proceedings of the Royal Society_, 1870 and 1872]

Thus, though it is true that sea-water steadily contracts as it cools
down to its freezing point, instead of expanding before it reaches its
freezing point as fresh water does, the truth has been steadily ignored
by even the highest authorities in physical geography, and the erroneous
conclusions deduced from their erroneous premises have been widely