Fig. 3.
1. Small specimen of Eozoon Canadense, weathered out from the containing rock, and showing its laminated structure.
2. Casts of irregular or acervaline chambers of upper part (magnified).
3. Surface of a cast of a flat chamber, showing its constituent chamberlets (magnified).
4. Section of casts of flat chambers (magnified). From the Laurentian of Canada.
Between the rocks which contain Eozoon and the next in which we find any abundant remains of life, there is a gap in geological history, either destitute of evidence of life or showing nothing materially in advance of Eozoon. In the Cambrian Age, however, we obtain a vast and varied accession of life. Here we find evidence that the sea swarmed with living creatures near akin to those which still inhabit it, and nearly as varied. Referring merely to leading groups, we have here the soft shellfishes and the worms, the ordinary shellfishes, the sea-stars, and the corals, with the sponges. In short, had we been able to drop our dredge into the Cambrian or Lower Silurian ocean, we should have brought up representatives of all the leading types of invertebrate life that exist in the modern seas—different, it is true, in details of structure from those now existing, but constructed on the same principles and filling the same places in nature.
If we inquire as to the history of this swarming marine life of the early Palæozoic, we find that its several species, after enduring for a longer or a shorter time, one by one became extinct and were replaced by others belonging to the same groups. Thus there is in each great group a succession of new forms, distinct as species, but not perceptibly elevated in the scale of being. In many cases, indeed, the reverse seems to be the case; for it is not unusual to find the successive dynasties of life in any one family manifesting degradation rather than elevation. New, and sometimes higher, forms, it is true, appear in the progress of time, but it is impossible, except by violent suppositions, to connect them genetically with any predecessors. The succession throughout the Palæozoic presents the appearance rather of the unchanged persistence of each group under a succession of specific forms, and the introduction from time to time of new groups, as if to replace others which were in process of decay and disappearance.
In the later half of the Palæozoic we find a number of higher forms breaking upon us with the same apparent suddenness as in the case of the early Cambrian animals. Fishes appear, and soon abound in a great variety of species, representing types of no mean rank, but, singularly enough, belonging, in many cases, to groups now very rare; while the commoner tribes of modern fish do not appear. On the land, batrachian reptiles now abound, some of them very high in the sub-class to which they belong. Scorpions, spiders, insects, and millipedes appear, as well as land-snails, and this not in one locality only, but over the whole northern hemisphere. At the same time, the land appears clothed with an exuberant vegetation—not of the lowest types nor of the highest, but of intermediate forms, such as those of the pines, the club-mosses, and the ferns, all of which attained in those days to magnitudes and numbers of species unsurpassed, and in some cases unequalled, in the modern world. Nor do they show any signs of an unformed or imperfect state. Their seeds and spores, their fruits and spore-cases, are as elaborately constructed, the tissues and forms of their stems and leaves as delicate and beautiful, as in any modern plants. So with the compound eyes and filmy wings of insects, the teeth, bones, and scales of batrachians and fishes; all are as perfectly finished, and many quite as complex and elegant, as in the animals of the present day (Figure 4).
Fig. 4.