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One-tenth natural size. Menevian beds. St. Immediately beneath the Lingula Flags there occurs a series of dark grey and black flags and slates alternating at the upper part with some beds of sandstone, the whole reaching a thickness of from 500 to 600 feet. These beds were formerly classed, on purely lithological grounds, as the base of the Lingula Flags, but Messrs.
Here, therefore, it may truly be said that we are entering upon a new domain of life in our retrospective survey of the past. The Tremadoc slates are very local, and seem to be confined to a small part of North Wales; and Professor Ramsay supposes them to lie unconformably on the Lingula flags, and that a long interval of time elapsed between these formations.
I refer to the well-known Lingula of the Lingula flags, lately, in consequence of some slight differences, placed in the new genus Lingulella. Practically, it belongs to the same great generic group as the Lingula, which is to be found at the present day upon your own shores and those of many other parts of the world.
If A.B. wishes to prove an alibi, it is of no use for him to get a thousand witnesses simply to swear that they did not see him in such and such a place, unless the witnesses are prepared to prove that they must have seen him had he been there. But the evidence that animal life commenced with the Lingula- flags, e.g., would seem to be exactly of this unsatisfactory uncorroborated sort.
In fact, the southern boundary marches with the northern limit of the Crockerville estate. The rich gold-bearing lode is situated on the western slope of the hill, and can be seen in all the three shafts which have been sunk. The formation of the hill seems in many respects to correspond with the Lingula flags at and near Clogau, Dolgelli, and Gogafau.
Some of the most ancient animals, as the Nautilus, Lingula, etc., do not differ much from living species; and it cannot on our theory be supposed, that these old species were the progenitors of all the species belonging to the same groups which have subsequently appeared, for they are not in any degree intermediate in character.
For all the species of the same group, however long it may have lasted, are the modified descendants one from the other, and all from a common progenitor. In the genus Lingula, for instance, the species which have successively appeared at all ages must have been connected by an unbroken series of generations, from the lowest Silurian stratum to the present day.
Davidson on the British Brachiopoda, illustrates, in the first place, the tendency of certain generic forms in this division of the mollusca to be persistent throughout the whole range of geological time yet known to us; for the four genera, Rhynchonella, Crania, Discina, and Lingula, have been traced through the Silurian, Devonian, Carboniferous, Permian, Jurassic, Cretaceous, Tertiary, and Recent periods, and still retain in the existing seas the identical shape and character which they exhibited in the earliest formations.
They have yielded about forty species of fossils, of which six only are common to the overlying Tremadoc rocks, but the two formations are closely allied by having several characteristic "primordial" genera in common. According to Mr. Belt, who has devoted much attention to these beds, there are already palaeontological data for subdividing the Lingula Flags into three sections.
Species of the genus Lingula, for instance, must have continuously existed by an unbroken succession of generations, from the lowest Silurian stratum to the present day. We have seen in the last chapter that the species of a group sometimes falsely appear to have come in abruptly; and I have attempted to give an explanation of this fact, which if true would have been fatal to my views.
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