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We entered on a zone where the whole sea was covered with a prodigious quantity of medusas. The vessel was almost becalmed, but the mollusca were borne towards the south-east, with a rapidity four times greater than the current. Their passage lasted near three quarters of an hour. We then perceived but a few scattered individuals, following the crowd at a distance as if tired with their journey.
In order to account for ancient life, that passed away, as far back as the carboniferous age, it is claimed that millions of years passed before that age began. But here are the very first species of mollusca in the more recent clays unchanged, and here are the same little animals that floored so much territory in the bygone with chalk.
We may picture to ourselves the primitive ancestor of mollusks as a worm having the short and broad form of the turbellaria, but much thicker or deeper vertically. A fuller description can be found in the "Encyclopædia Britannica," Art., Mollusca. It was hemi-ovoid in form. It had apparently the perivisceral cavity and nephridia of the schematic worm, and a circulatory system.
Science records with unerring certainty the progress of the earth, and of animal life, from the lowest existences in the mollusca and polypi, up to the superlatively complicated, and delicate structure of man, tracing it step by step, until it is finished in the noblest work of God, a human body coupled with an immortal soul!
We have seen that the highly remarkable fact of the absence of any EXTENSIVE formations containing recent shells, either on the eastern or western coasts of the continent, though these coasts now abound with living mollusca, though they are, and apparently have always been, as favourable for the deposition of sediment as they were when the tertiary formations were copiously deposited, and though they have been upheaved to an amount quite sufficient to bring up strata from the depths the most fertile for animal life can be explained in accordance with the above proposition.
In the last edition of Sir Charles Lyell's "Elementary Geology" it is stated, on the authority of a former President of this Society, the late Daniel Sharpe, that between 30 and 40 per cent. of the species of Silurian Mollusca are common to both sides of the Atlantic.
In the white substratum of sand, gently shelving far under the sea, there was not a sufficiency of organic matter to have afforded food for fish even for the lower organisms of mollusca. Undoubtedly were these castaways alone; as much so as if their locality had been the centre of the Atlantic, instead of its coast!
The largest number of fossils have been collected from the tuffs and conglomerates and some beds of limestone in the island of Baixo, off the southern extremity of Porto Santo. They amount in this single locality to more than sixty in number, of which about fifty are mollusca, but many of these are only casts.
Every geologist is aware that great as have been the geographical changes in the northern hemisphere since the commencement of the Glacial Period, there having been submergence and re-emergence of land to the extent of 1000 feet vertically, and in the temperate latitudes great vicissitudes of climate, the marine mollusca have not changed, and the same drift which had been carried down to the sea at the beginning of the period is now undergoing a second transportation in the same direction.
Strange as it may appear at first sight, some of the higher forms of the mollusca show signs of a rudimentary vertebra, and science has hazarded the opinion that the sea-squirts and similar creatures were descended from some ancestor from whom also descended the vertebrate animals, of which man is the highest form known today on this planet.
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