Fig. 14.—Fragment of Cryptozoon, Grand Cañon, Arizona.
Photograph from a specimen presented by Dr Walcott to the Peter Redpath Museum.
In a thick series of pre-Cambrian beds in the Colorado Cañon in the Western United States, Walcott has found a small roundish shell of uncertain affinities,[8] a species of Hyolithes, probably a swimming sea-snail or Pteropod, a small fragment which may possibly have belonged to a Trilobite, and some laminated forms which, if organic, are related to the Cryptozoon already mentioned ([Fig. 14]).
[8] Discinoid or Patelloid.
The Kewenian series of Lake Superior has yielded no fossils, but the pipestone beds of Minnesota, supposed to be about the same age, have afforded a small bivalve shell allied to Lingula;[9] and the black shales of the head of Lake Superior contain some impressions supposed to be trails of animals.[10]
[9] Winchell.
[10] Selwyn and Matthew.
It has been a question whether the beds above referred to should be regarded as a downward continuation of the Cambrian, or as the upper part of an older system. Matthew, whose opinion on such a subject is of the highest authority, regards them as a distinct system, but as belonging, with the Cambrian, to the great Palæozoic Period. Van Hise, and some other United States authorities, would separate them even from the Palæozoic, and unite them with the underlying Huronian, as representing a "Proterozoic" or "Algonkian" Period. This is merely a matter of classification, necessarily more or less arbitrary; but I believe the facts to be stated subsequently show that it will be best to unite the Etcheminian and its equivalents with the Palæozoic, and to place the groups lower than this in one great division, equivalent to Palæozoic, and for which many years ago I proposed the name "Eozoic," or that of the Dawn of Life.
Having thus hastily glanced at the slender fauna of the rocks immediately below the Cambrian, we may now proceed to inquire a little more in detail into its true value and import as leading toward the beginning of life. I have already referred to the apparently sudden drop in the number of groups and of species below the base of the Cambrian, and have hinted that this may be an effect of temporary local conditions of deposit or of defective information. Another fact that strikes us is the diverse and miscellaneous character of the fossils that remain to us; and this would suggest that we are either dealing with a mere handful picked at random, as it were out of a richer fauna, or that in the beginning of things the gaps and missing links between different forms of life were even more pronounced than at present. This, however, would be likely to occur if the plan of creation was to represent at first different types, with few forms in each; to produce, in short, a sort of type collection representing the whole range of organization by a few characteristic things rather than to give a complete series, with all the intermediate connections. Such a mode of introduction of life is not à priori improbable, however at variance with some prevalent hypotheses.
Beginning with the higher Invertebrates, we must not conclude that we have altogether lost the Trilobites. The fragments referred to this group may represent at least a few species, and it would be very interesting to know more of these as to their relations to their successors, and whether they are tending to lower or more embryonic forms. The bivalve Crustaceans (Ostracods) may be regarded as inferior in rank to the Trilobites, but are still very complex, and specialized animals and a specimen silicified in such a manner as to show the interior organs testified that, as far back as the Carboniferous at least, these creatures were as highly organized as at present,[11] while their generally larger size in the earlier formations tends to show that they have rather been degenerating in the lapse of geological time.
[11] Palæocypris Edwardsi, Brougniart, Coal Formation of St. Etienne, France.