Take, for instance, the similarity above alluded to between the fauna of the Arctics and that of the Alps, certainly showing a direct relation between climatic conditions and animal and vegetable life. Yet even there, where the shades of specific difference between many animals and plants of the same class are so slight as to battle the keenest investigators, we have representative types both in the Animal and Vegetable Kingdoms as distinct and peculiar as those of widely removed and strongly contrasted climatic conditions. Shall we attribute the similarities and the differences alike to physical causes? Compare, for example, the Reindeer of the Arctics with the Ibex and the Chamois, representing the same group in the Alps. Even on mountain-heights of similar altitudes, where not only climate, but other physical conditions would suggest a recurrence of identical animals, we do not find the same, but representative types. The Ibex of the Alps differs, for instance, from that of the Pyrenees, that of the Pyrenees from those of the Caucasus and Himalayas, these again from each other and from that of the Altai.
But perhaps the most conclusive proof that we must seek for the origin of organic life outside of physical causes consists in the permanence of the fundamental types, while the species representing these types have differed in every geological period. Now what we call typical features of structure are in themselves no more stable or permanent than specific features. If physical causes, such as light, heat, moisture, food, habits of life, etc., acting upon individuals, have gradually in successive generations changed the character of the species to which they belong, why not that of the class and the branch also? If we judge this question from the material side at all, we must, in order to judge it fairly, look at it wholly from that point of view. If these specific changes are brought about in this way, it is because external causes have positive permanent effects upon the substances of which animals are built: they have power to change their hair, to change their skin, to change certain external appendages or ornamentations, and any other of those ultimate features which naturalists call specific characters. Now I would ask what there is in the substances out of which class characters are built that would make them less susceptible to such external influences than these specific characters. In many instances the former are more delicate, more sensitive, far more fragile and transient in their material nature than the latter. And yet never, in all the chances and changes of time, have we seen any alteration in the mode of respiration, of reproduction, of circulation, or in any of the systems of organs which characterize the more comprehensive groups of the Animal Kingdom, although they are quite as much under the immediate influence of physical causes as those structural features which have been constantly changing.
The woody fibre of the Pine-trees has had the same structure from the Carboniferous age to this day, while their mode of branching and the forms of their cones and leaves have been different in each period according to their respective species. The combination of rings, the structure of the wings, and the articulations of the legs are the same in the Cockroaches of the Carboniferous age as in those which infest our ships and our dwellings to-day, while the proportion of their parts is on quite another scale. The tissue of the Corals in the Silurian age is identical in chemical combination and organic structure with that of the Corals of our modern reefs, and yet the extensive researches upon this class for which we are indebted to Milne Edwards and Haime have not revealed a single species extending through successive geological ages, but show us, on the contrary, that every age has had its own kinds, differing among themselves in the same way as those of the Gulf of Mexico differ now from those of the Indian Ocean and the Pacific. The scales of the oldest known fishes in the Silurian beds have the same microscopic structure as those of their representative types today, and yet I have never seen a single fossil fish presenting the same specific characters in the successive geological epochs. The teeth of the oldest Sharks show the same microscopic structure as those of the present time, and we do not lack opportunities for comparison, since the former are as common in the mountain-limestone of Ireland as are those of the living Sharks on any beach where our fishermen boil them for the sake of their oil, and yet the Sharks appear under different generic and specific forms in each geological age.
But without multiplying examples, which might be adduced ad infinitum, to show permanence of type combined with repeated changes of species, suffice it to say, that, while the general features in the framework of the organic world and the materials of which that framework is built, though quite as subject to the influence of physical external circumstances as any so-called specific-features, have remained perfectly intact from the beginning of Creation till now, so that not the smallest difference is to be discerned in these respects between the oldest representatives of the oldest types in the oldest Silurian rocks and their successors through all the geological ages up to the present day, the species have been different in each epoch. It is surely a fair question to ask the advocates of the transmutation theory, whether they attribute to physical laws the discernment that would lead them to change the specific features, but to respect all those characters by which the higher structural combinations of the Animal Kingdom are preserved without alteration,—in other words, to maintain the organic plan, while constantly diversifying the mode of expressing it. If so, it would perhaps be as well to call them by another name, since they show all the comprehensive wisdom of an intelligent Creator. Until they can tell us why certain features of animals and plants are permanent under conditions which, according to their view, have power to change certain other features no more perishable or transient in themselves, the supporters of the development theory will have failed to substantiate their peculiar scientific doctrine.
But this discussion has led us far away from our starting point, and interrupted our walk along the Silurian beach; let us return to gather a few specimens there, and compare them with the more familiar ones of our own shores. I have said that the beach was a shelving one, and covered of course with shoal waters; but as I have no desire to mislead my readers, or to present truths as generally accepted which are still subject to dispute, I would state here that the parallel ridges across the State of New York, considered by some geologists as the successive shores of a receding ocean, are believed by others to be the inequalities on the bottom of a shallow sea. Not only, however, does the general character of these successive terraces suggest the idea that they must have been shores, but the ripple-marks upon them are as distinct as upon any modern beach. The regular rise and fall of the water is registered there in waving, undulating lines as clearly as on the sand-beaches of Newport or Nahant; and we can see on any one of those ancient shores the track left by the waves as they rippled back at ebb of the tide thousands of centuries ago. One can often see where some obstacle interrupted the course of the water, causing it to break around it; and such an indentation even retains the soft, muddy, plastic look that we observe on the present beaches, where the resistance made by any pebble or shell to the retreating wave has given it greater force at that point, so that the sand around the spot is soaked and loosened. There is still another sign, equally familiar to those who have watched the action of water on a beach. Where a shore is very shelving and flat, so that the waves do not recede in ripples from it, but in one unbroken sheet, the sand and small pebbles are dragged and form lines which diverge whenever the water meets an obstacle, thus forming sharp angles on the sand. Such marks are as distinct on the oldest Silurian rocks as if they had been made yesterday. Nor are these the only indications of the same fact. There are certain animals living always upon sandy or muddy shores, which require for their well-being that the beach should be left dry a part of the day. These animals, moving about in the sand or mud from which the water has retreated, leave their tracks there; and if, at such a time, the wind is blowing dust over the beach, and the sun is hot enough to bake it upon the impressions so formed, they are left in a kind of mould. Such trails and furrows, made by small Shells or Crustacea, are also found in plenty on the oldest deposits.
Admitting it, then, to be a beach, let us begin with the lowest type of the Animal Kingdom, and see what Radiates are to be found there. There are plenty of Corals, but they are not the same kinds of Corals as those that build up our reefs and islands now. The modern Coral animals are chiefly Polyps, but the prevailing Corals of the Silurian age were Acalephian Hydroids, animals which indeed resemble Polyps in certain external features, and have been mistaken for them, but which are nevertheless Acalephs by their internal structure; for, instead of having the vertical partitions dividing the body into chambers, so characteristic of the Polyps, they are divided by tubes corresponding to the radiating tubes of the Acalephs proper, these tubes being themselves divided at regular distances by horizontal floors, so that they never run uninterruptedly from top to bottom of the body. I subjoin a woodcut of a Silurian Coral, which does not, however, show the peculiar internal structure, but gives some idea of the general appearance of the old Hydroid Corals. We have but one Acalephian Coral now living, the Millepore; and it was by comparing that with these ancient ones that I first detected their relation to the Acalephs. For the true Acalephs or Jelly-Fishes we shall look in vain; but the presence of the Acalephian Corals establishes the existence of the type, and we cannot expect to find those kinds preserved which are wholly destitute of hard parts. I do not attempt any description of the Polyps proper, because the early Corals of that class are comparatively few, and do not present features sufficiently characteristic to attract the notice of the casual observer.
[Illustration]
[Illustration]
Of the Echinoderms, the class of Radiates represented now by our Star-Fishes and Sea-Urchins, we may gather any quantity, though the old fashioned forms are very different from the living ones. I have dwelt at such length in a former article[A] on the wonderful beauty and variety of the Crinoids, or "Stone Lilies," as they have been called, from their resemblance to flowers, that I will only briefly allude to them here. The subjoined wood-cut represents one with a closed cup; but the number of their different patterns is hardly to be counted, and I would invite any one who questions the abundant expression of life in those days to look at some slabs of ancient limestone in the Zoölogical Museum at Cambridge, where the stems of the Crinoids are tangled together as thickly as sea-weed on the shore. Indeed, some of our rock-deposits consist chiefly of the fragments of their remains.
[Footnote A: See Methods of Study in Natural History, Atlantic
Monthly, No. LVII., July, 1862.]