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among the most widely extended groups, constantly represented in closely allied, if not identical specific forms. In other words, they are the most cosmopolitan of existing Mollusca; they are gregarious and often live in great depths of water, but lastly they have considerable vertical range, and are greatly similar to one another in their development in time. Some, no doubt, of the highest Mollusca, nearly related to the common squid or cuttle-fish, are also very widely spread, but this arises from the free-swimming habits of the animal, and is therefore to be considered as having a different cause from that which obtains in the former case, where the animal, at least in the fullgrown state, is permanently attached to some submarine body.
No animals, again, at the present day, are more widely distributed than those which secrete and deposit in various ways solid calcareous matter; and of all these it would be difficult to find any that have greater influence than some of the smaller zoophytes. It is just these animals also whose remains are distributed through rocks of various ages, and which, therefore, seem to determine least effectually, in time as well as space, any important point with regard to the true geological position of a containing rock.
If, leaving the Invertebrata, we examine the various groups of vertebrate animals, a nearly similar result is obtained. Certain groups of fishes now characterise certain limited districts, and this without our being able to discover any reason for it. Of these groups some exhibit much higher organization than others, and present marked differences of habit and structure, while some, on the other hand, are more
widely distributed and more nearly cosmopolitan. On our own coast, again, we should find buried at moderate depths, and at no great distance from the coast-line, a multitude of animals, exhibiting probably but few decided indications of the vicinity of land except within the range of comparatively shoal water. On the coast of America, on the contrary, there would be extensive deposits in the open sea, at a distance of several hundred miles from the coast, containing occasionally plants and animals floated down by the vast rivers of that continent, and conveyed along on the surface by the river stream and the marine currents. On the coast of Asia we should probably have striking indications of the existence of animals of high organization; while on the coast of Australia there would be scarcely any mark of higher conditions than those which are known in the oolitic rocks. Lastly, in those distant parts where the ocean is broad and deep, and the islands small and scattered, there might only be seen the remains of imperfectly organized Foraminifera, mixed perhaps with a few Radiata, such as the deepsea urchins, and fragments of some free-swimming animals.
Now it is important to consider these great differences, because they lead us to the only true means of judging with regard to geological phenomena. Any one visiting in succession Australia, South America, Europe, South Africa, and Asia, and looking at the animal kingdom without taking man into account, might come to the conclusion, that, in point of development and complexity of organization, there was on the whole a distinct advance in the scale
of beings; or that, in other words, the indigenous mammalian animals or quadrupeds existed in a condition less removed from that of birds and reptiles in Australia than in South America, in South America than in Europe, in Europe than in South Africa, and in South Africa than in Asia, since, in the first-named district, he would find the marsupial or pouched animals exclusively present, in the next the edentates most characteristic,* in the third the ruminants, in the fourth the pachyderms and carnivorous animals, and in the last the pachyderms, Carnivora, and monkeys. Would he for this reason, however, be justified in concluding that in either case the kind of progress exhibited in Nature's works would gradually bring out edentates from marsupials, ruminants from edentates, carnivora from ruminants, or monkeys from carnivora? We may safely assert that such a conclusion would be false; nor is there, in any case, the shadow of probability that progress or development of this nature has ever existed. And when, in examining rocks of different age, we discover marsupials in the oolite, what is the actual evidence with regard to the other groups of quadrupeds? It is simply this, that in the next newer beds in which the remains of quadrupeds appear, there are marsupials, ruminants, pachyderms, carnivores, and monkeys, indiscriminately mixed together; and with regard to the three latter groups, it is not easy to determine which of them was the truly characteristic
*In South America there is now not one indigenous species of the hollow-horned ruminants (ox, sheep, goats, &c.), and the pachyderms and Carnivora are few in number of species, and of small size. This was not the case, however, during the Tertiary period.
one; for, though the remains of pachyderms seem to be more abundant than any others, this may well be a result of their peculiar habits and the swampy condition of the land near where the beds were deposited, and we know that such animals are at present, and may well suppose that they always were, associated with numerous and powerful species of Carni
Besides, however, these facts with regard to the present distribution of quadrupeds, we find that the distribution of the groups in times not long past exhibited a strictly analogous class of results. In South America, for instance, the horse and several other pachyderms, several hollow-horned ruminants, and some carnivores of larger proportions than now exist, anciently inhabited the country and were contemporaries of the gigantic edentates. There is some evidence also to prove that North and South America were formerly united much more directly than they are now, the more highly organized group seeming to have been destroyed in the southern part of the continent, but having been retained in the northern. So also in England and western Europe we find fossil monkeys, and indications of vast multitudes of large Carnivora and pachyderms, although these animals have there been for a long time nearly or altogether extinct.
The result that we are forced to arrive at from such considerations as these is, that climate and atmospheric conditions, the consequence probably of differences in the quantity of land above water in certain districts, in the relative position, the extension, and the level of the landl, combined, it may be,
with other causes concerning which it would be idle to speculate, had far more to do with the extent and the true nature of the organic forms, than any incomplete development of the mammalian class.
The notion of a true progressive development, the geologist therefore, as well as the zoologist and botanist, must except against. There are no good grounds for believing in its existence in any case at present; and the history of the past is decidedly opposed to the idea of such a plan having ever been in operation.
Nature, in fact, will not allow herself to be tortured into our systems, nor will she adapt herself to the procrustean bed of any system-maker amongst us. The infinite ramifications of life, the thrusting in, as it were, in every spot where life is possible, of those animals and vegetables best fitted to exist under such circumstances; these of themselves are sufficiently important facts, and speak clearly enough to check presumptive and hasty generalizations, if indeed it were possible to check this natural tendency to advance too hastily to conclusions. At all degrees of temperature, from the surface of snow to the boiling water of hot springs; in all soils, from the rich land of the tropics to the barren desert; on the whitened surface of pure salt and on the naked rock on the mountain summit; in all degrees of light, from the full glare of sunshine to the darkest recesses of the rocky cavern; in air and in water; upon the earth, and even beneath the earth; at all times, from the first introduction of living beings until now-we find pressing in on every side abundant evidence of this marvellous fact, this perpetual miracle. We are