These are not solitary cases. The student of fossils meets with them at every turn; and if he possesses the taste and imagination of a true naturalist, cannot fail to be impressed with them.
To turn to a later but very ancient period, what can be more astonishing than those first air-breathing vertebrates of the Coal formation referred to in a previous chapter, with all their special arrangements for an aërial habitat? I have mentioned their footprints, and when we see the quarrymen split open a slab of sandstone and expose a series of great plantigrade tracks, not unlike those of a human foot, with the five toes well-developed, we are almost as much astonished as Crusoe was when he saw the footprints on the sand. Crusoe inferred the presence of another man in his island; we infer the earliest appearance of an air-breathing vertebrate and the pre-human determination of the form and number of parts of the human foot and hand, to appear in the world long ages afterward. We see also that already that decimal system of notation which we have founded on the counting of our ten fingers was settled in the framework of most unmathematical Batrachians. It has approved itself ever since as the typical and most perfect number of parts for such organs.
If sceptically inclined, we may ask, Why five rather than four or six? In the case of man we see that individuals who have lost one finger have the use of the hand impaired, while the few who happen to have six do not seem to be the better. How it was with the old Batrachians we do not know; but it is certain that if we could have amputated the claw-bearing little toe of Sauropus unguifer, or the reflexed little toe of Cheirotherium, we should have much injured their locomotive power.
The vegetable kingdom is full of similar examples of the early settlement of great questions. Perhaps nothing is more marvellous than the power of the green cells of the leaf as workers of those complex and inimitable chemical changes whereby out of the water, carbon dioxide and ammonia of the soil and the atmosphere, the living vegetable cell, with the aid of solar energy, elaborates all the varied organic compounds produced by the vegetable kingdom. Yet this seems all to have been settled and perfected in the old Silurian period, long before any kind of plant now living was on the earth. Perhaps in some form it existed even in the Laurentian age, and was instrumental in laying up its great beds of carbon. So all that is essential in plant reproduction, whether in that simpler form in which a one-celled spore is the reproductive organ, or in that more complex form in which an embryo plant is formed in the seed, with a store of nourishment laid up for its sustenance.
These arrangements were obviously as perfect in the great club mosses and pines of the Devonian and Carboniferous as they have ever been since, and we have specimens so preserved as to show their minute parts just as well as in recent plants. The microscope also shows us that the contrivances for thickening and strengthening the woody fibres and trunk of the stem by bars or interrupted linings of ligneous matter, so as to give strength and at the same time permit transudation of sap, were all perfected, down to their minutest details, in the oldest land plants. It is true that flowers with gay petals and some of the more complicated kinds of fruit are later inventions, but the additions in these consist mainly of accessories. The essentials of vegetable reproduction were as well provided for from the first.
The same principle applies to many of the leading forms and types of life, considered as genera or species. While some of these are of recent introduction, others have continued almost unchanged from the remotest ages. Such creatures as the Lingulæ, some of the Crustaceans and of the Mollusks, the Polyzoa and some Corals have remained with scarcely any change throughout geological time, while others have disappeared, and have been replaced by new types.
We began this chapter with a consideration of the permanence of continental areas, and may close with a reference to the same great fact in connection with the continuity of life. Whether with some we attach more importance to the support of the continents by lateral pressure and rigidity, or with others to what may be termed flotation, by virtue of their less density, as compared with that of the lower parts of the earth; there can be little doubt that both principles have been applied, and that both admit of some vertical movement. Thus the stability of the continents is one of position rather than height, and their internal plateaus as well as their partially submerged marginal slopes have undergone great and unequal elevations and depressions, causing most important geographical changes. Among these are the formation of connecting bridges of shoals, islands, or low land, connecting the continental masses at different periods, and permitting migrations of shallow-water animals and even of denizens of the land. The facts adduced in previous pages are sufficient to show connections across the north of the Atlantic at intervals reaching from the Cambrian to the Modern.
The conclusion of the whole matter is that there is a fixity and unchangeableness in determinations and arrangements of force just as much as in natural laws; and that while God has made everything beautiful in its time He has also made everything beautiful and useful in some sense for all time. With all this, while the great principles and modes of operation remain unchanged, there is ample scope for development, modification and adaptation to new ends, without deviation from essential properties and characters. It is a wise and thoughtful philosophy which can distinguish what is fixed and unchangeable from that which is fluctuating and capable of development. Until this distinction is fully understood, we may expect one-sided views and faulty generalizations in our attempts to understand nature.
References:—"The Chain of Life in Geological Times." London. New Species of Fossil Sponges from the Quebec Group at Little Metis. Trans. Royal Society of Canada, 1889. Fossil Fishes from the Lower Carboniferous of New Brunswick. Canadian Naturalist, "Acadian Geology," 1855, and later editions to 1892. London and Montreal. "The Story of the Earth," 1872 and later editions to 1891. London.