Some Salient Points in the Science of the Earth

by Sir John William Dawson

The same fact appears in geological time. The periods represented by the older Palæozoic rocks have been termed ages of invertebrates, and they might also be termed ages of aquatic animals. It is only gradually, and as it were with difficulty, that animals living in the less congenial element of air are introduced—at first a few scorpions and insects, later, land snails and amphibian reptiles, later still, the higher reptiles and the birds, and last of all the higher mammalia.

We need not wonder at this, for the conditions of life with reference to support, locomotion, and vicissitudes of temperature are more complex and difficult in air, and require more complicated and perfect machinery for their maintenance. Thus it was that probably half of the whole history of our earth had passed away before the land became the abode of any large number and variety of animals; while it was only about the same time that the development of the vegetable kingdom became so complete as to afford food and shelter for air-breathers.

It is also worthy of note that it is only in comparatively recent times that we have been able to discover the oldest air-breathing animals, and geologists long believed that the time when animals had existed on the land was even shorter than it had actually been. This arose in part from the infrequency and rarity of preservation of the remains of the earliest creatures of this kind, and perhaps partly from the fact that collectors were not looking for them.

That there was dry land, even in the Cambro-Silurian period, we know, and can even trace its former shores. In Canada our old Laurentian coast extends for more than a thousand miles, from Labrador to Lake Superior, marking the southern border of the nucleus of the American continent in the Cambrian and Cambro-Silurian periods. Along a great part of this ancient coast we have the sand flats of the Potsdam Sandstone, affording very favourable conditions for the imbedding of land animals, did these exist; still, notwithstanding the zealous explorations of the Geological Survey, and of many amateurs, no trace of an air-breather has been found. I have myself followed the oldest Palæozoic beds up to their ancient limits in some localities, and collected the shells which the waves had dashed on the beach, and have seen under the Cambro-Silurian beds the old pre-Cambrian rocks pitted and indented with weather marks, showing that this shore was then gradually subsiding; yet the record of the rocks was totally silent as to the animals that may have trod the shore, or the trees that may have waved over it. All that can be said is that the sun shone, the rain fell, and the wind blew as it does now, and that the sea abounded in living creatures. The eyes of Trilobites, the weathered Laurentian rocks, the wind ripples in the Potsdam sandstone, the rich fossils of the limestones, testify to these things. The existence of such conditions would lead us to hope that land animals may yet be found in these older formations. On the other hand, the gradual failure of one form of life after another, as we descend in the geological series, and the rarity of fishes and land plants in the Silurian rocks and their absence from the Cambrian, might induce us to believe that we have here reached the beginning of animal life, and have left far behind us those forms that inhabit the land.

Even in the Carboniferous period, though land plants abound, air-breathers are not numerous, and most of them have only been recently recognised. We know, however, with certainty that the dark and luxuriant forests of the coal period were not destitute of animal life. Reptiles[122] crept under their shade, land snails and millipedes fed on the rank leaves and decaying vegetable matter, and insects flitted through the air of the sunnier spots. Great interest attaches to these creatures; perhaps the first-born species in some of their respective types, and certainly belonging to one of the oldest land faunas, and presenting prototypes of future forms equally interesting to the geologist and the zoologist.

[122] I shall use the term reptile here in its broad, popular sense, as including Batrachians as well as reptiles proper.

It has happened to the writer of these pages to have had some share in the finding of several of these ancient animals. The coal formation of Nova Scotia, so full in its development, so rich in fossil remains, and so well exposed in coast cliffs, has afforded admirable opportunities for such discoveries, which have been so far improved that at least twenty-five out of the not very large number of known Carboniferous land animals have been obtained from it.[123] The descriptions of these creatures, found at various times and at various places, are scattered through papers ranging in date from 1844 to 1891,[124] and are too fragmentary to give complete information respecting the structures of the animals, and their conditions of existence.

[123] It appears that about a hundred species of Carboniferous reptiles have been recognised on the continent of Europe, in Great Britain, and in the United States. They belong to a number of distinct types, all, however, being of batrachian affinities.

[124] Papers by Lyell, Owen, and the author, in the Journal of the Geological Society of London, i. ii. ix. x. xi. xvi. xvii. xviii.; "Acadian Geology," by the author; Papers in Trans. Royal Society of London, Am. Jl. of Science, and Geological Magazine.

Footprints.