There is another point suggested by the occurrence of mollusca in the Carboniferous system, to which it may be well to refer, namely, the curious, and as yet not wholly understood fact, that the range of animals in time is in some way proportionate to their range in space. In other words, it often happens (so often, indeed, as apparently to indicate a law) that the more widely diffused a genus is found to be at the present day, the farther back can we trace its remains into the geological ages. This fact probably depends upon causes, many of which are still unknown to us; but the following remarks may help the reader to a notion of the general bearings of the subject.[34] [Footnote 34: The law is more especially exemplified by the mollusca, but it may eventually be found to characterize other classes. We, perhaps, see traces of it in the present distribution of the two most ancient orders of icthyic life--the placoids and ganoids.] In the profounder recesses of the ocean, the temperature remains more or less uniform all over the globe.[35] In these undisturbed regions there occur, along with corals and other humble animals, many kinds of mollusca, such as terebratulæ, craniæ, scissurellæ, &c. These are very generally found not to be confined to one province or limited district, but to flourish in every sea from Hudson's Bay to Hindustan. One of the causes of this wide distribution is the uniformity of temperature that characterizes the depths in which they live. They can migrate from one ocean to another, from the torrid zone to the polar circle, without experiencing any destructive change in the thermal conditions of their element. And provided only they meet with no barrier in the form of a lofty submarine mountain chain or profound abyss, and can secure the requisite food in their journey, we know no reason why some of these shells may not thus extend themselves over wide areas. Of the two species of _rhynconella_ now living, one inhabits the depths of the icy sea, the other enjoys the warmer waters that lave New Zealand. The species, in this case, seem (for the fact cannot yet be accepted as fully proved) to occupy a more limited area, while the genus has a larger range. [Footnote 35: The stratum of constant temperature runs in a wave-like form from pole to pole. In the arctic and antarctic oceans it is found at a depth of 4500 feet, whence it slopes upwards so as to reach the surf ice at the temperate zone on both sides of the equator. It then gradually sinks down in the warmer regions, till at the equator it is 7200 feet below the sea-level. There are thus one tropical and two polar basins separated by two wave-like circles, or, as a geologist would say, three synclinal troughs separated by two anticlinal ridges.]