II
LIFE IN THE EARLY CAMBRIAN
I
IN the old Chaldean fable of the descent of Ishtar into Hades, to recover her lost Tammuz, at each successive gate of the lower regions she is stripped of some of her ornaments and garments, till at length she has to appear naked and unadorned in the presence of the lord of the Nether World. So in our descent from the surface on which men live, through the successive rocky layers of the earth's crust, we leave behind, one by one, all the higher forms of life with which we are familiar; but there still remain to us our six groups of aquatic invertebrates, in the guise, it is true, of species and genera now unknown in a living state, yet well represented as far down as the lower part of the Cambrian. Let us now suppose that we take our stand on the shores of the Cambrian sea, or cast our dredge into its waters in search of these old animals; though we can only actually do so by painfully hammering and chiselling them out of their rocky tombs, and this often in fragments which must be put together before we can fully realize the forms and structures of the animals to which they belonged.
We may pause here, however, to remark that neither the geographical nor climatal conditions of the earth at this early time were similar to these with which we are now familiar. The marine animals of the Cambrian have left their remains in beds of sediment, which now constitute rocks forming parts of our continents remote from the sea, and much elevated above its level, showing that large areas, then under the ocean, are now dry land; while there is no good evidence that the sea and land have changed places. The facts rather indicate that the continents have extended their area at the expense of the ocean, which has, however, probably increased in depth. In evidence of these statements, I need only mention that some of the oldest rocks in the Scottish and Welsh hills, in Scandinavia, in Russia and in Bohemia, are rich in Cambrian marine fossils.
Fig. 1.—Olenellus Thompsoni, Hall.
A characteristic Trilobite of the Lower Cambrian in North America. After Walcott and specimen in Peter Redpath Museum.
In America, in like manner, such rocks are found on the flanks of the Appalachians, in New Brunswick, and in Newfoundland, in the table-land of Colorado and in the Rocky Mountains. In point of fact, a map of the Northern Hemisphere at this period would show only a limited circumpolar continent with some outlying islands to the south of it, and shallows stretching across the northern part of the areas of the present Atlantic and Pacific Oceans. The great ocean, however, thus extending over most of the temperate and tropical parts of the Northern Hemisphere, was probably also more muddy and shallow than that of modern times. The surface temperature of this vast ocean was also, it is probable, more uniform than that of the modern sea, while even its profounder depths or abysses would have more earth-heat than at present. Thus we may, without hesitation, affirm that in this early age the conditions for the introduction of swarming marine life of low grade, and its extension over the whole earth, were at a maximum.
Let us inquire, then, what these old Cambrian seas actually produced, more especially in the early portions of that ancient and probably protracted time.
The most highly organized type of which we have any certain evidence is that of the Crustacea, the group to which our modern lobsters and crabs belong, and its most prominent representatives are the trilobites (Figs. [1], [2]), so called from the three lobes into which the body is divided. These creatures are indeed remarkable for the twofold property of bilateral symmetry, and fore and aft jointed structure, both based on the number three. From front to rear we have a large head, usually with well-developed eyes and oral organs, a middle or thoracic part composed of a series of movable segments, and a tail-piece sometimes small, sometimes nearly as large as the head. Transversely, the body is divided into a central and two lateral lobes, which can be seen in the head, the thorax, and usually in the tail as well. The organization of these animals must have been as complex as that of most existing Crustaceans. Their nerve system must have been well developed; a vast number of muscles were required to move the different parts of the trunk, and the numerous and complex limbs which have been observed in some of the species, and no doubt were possessed by all. Their digestive and circulatory organs must have been in proportion to the complexity of their locomotive organs.