[33] It must be admitted, however, that not a few of the identifications already made are somewhat suspicious The natural tendency is to perceive resemblances—a tendency which even the most rigid science sometimes fails to control.
The geological ages saw many strange types of creation. One era, in especial, furnished reptiles which united in their structure the snout of the porpoise, the head of the lizard, the teeth of the crocodile, the paddles of the whale, and the backbone of the fish. Some displayed the long pliant neck of the swan, and others careered through the air on wings like those of the bat. But the molluscous tribes have never exhibited such aberrant forms. The existing classes and orders of the naturalist are still the same as those which nourished during the successive geological periods. Hence their value as evidence of physical changes in the ancient world. Hence, too, the conviction, forced upon the mind of the observer, that the conditions for the support of life never deviated much from those now in operation; that in place of all the varied beauty of the world having arisen for the use of man, it existed millions of years ere the breath of life had been breathed into his nostrils; that in fine, man is but a new-comer, a creation of yesterday.
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]
[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.
[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.
Now, a genus widely diffused, and capable of enduring great differences in the temperature and other conditions of the ocean, would probably suffer least from any great physical changes. If all the sea at one locality were converted into land, the genus would be driven into other districts, and thrive as abundantly as ever; or, even supposing that it should become locally extinct, it would still be abundantly represented in other oceans of the globe. In the course of many ages, after many such slow revolutions in the configuration of land and sea, the genus might perhaps become greatly reduced in numbers, until at length some final elevation of the sea-bed, or other change, might cause its total extinction. In the rhynconella, we perhaps see one of these genera in its last stage. Any great change in northern latitudes would probably destroy the arctic species, and a similar change around New Zealand might gradually extinguish the southern one.
Looking, then, from this point of view into the past history of life upon our planet, we see that such extinctions have often taken place. At first, many of these widely-diffused genera were created. They were represented by a large number of species as well as individuals, and ranged over all the oceans of the globe; but in tracing out their history, we mark one species after another passing away. Some of them lived for but a comparatively short period; others came in with the beginning and saw out the end of an entire geological system; but of all these early species there is not now a single one extant, though some of the genera still inhabit our seas. It is plain, therefore, that but for the operation of another principle, all the genera, too, would ere this have become extinct, for the whole can contain no more than the sum of its parts; and if these parts are destroyed the whole must perish simultaneously. As the species of certain genera died out, however, their places were from time to time filled up with new ones, yet the rate of increase became ever less and less than the rate of decrease, so that the numbers of such genera grew fewer with every successive period, and have reached their minimum in existing seas. There are instances, however, in which this ratio was reversed, the list of added species continually outnumbering that of the extinct, till the genus reached its maximum, when it either continued at that stage till the present day, or began slowly to decline.
In the physical world around us, we behold a perpetual strife between the two great principles of renovation and decay. Hills are insensibly crumbling into valleys; valleys are gradually cut down, and their debris transported to the sea. Our shores bear witness to the slow but ever onward march of the ocean, whether as shattered cliff's worn by the incessant lashing of the surge, or as sand-banks and submerged forests that represent the wolds and holms of our forefathers. We mark, too, how the sediment thus borne into the main is sowing
"The dust of continents to be;"