Note 7 — Chapter XIII · ANIMAL LIFE IN THE SEA
Reading edition
It does not follow that these creatures are conscious of this pressure, any more than we are of the pressure upon us of the fourteen pounds to the square inch of our atmosphere. The point is that they do feel it when they rise upward to a point where the pressure is distinctly less, just as we are conscious of a difference when we ascend in a balloon or climb a very high mountain, and after a time we find that we cannot go any farther. Land animals therefore have a vertical limit to their distribution as well as sea animals, and for analogous reasons.—E. I.