Изображения страниц
PDF
EPUB

ORIGIN OF LIFE ON THE EARTH.

We have found by this mode of inquiry that the abundance of the forms of life in the sea has been very unequal at different periods, and that race has followed race so as to match the words of the poet

Augescunt aliæ gentes, aliæ minuuntur,
Inque brevi spatio mutantur sæcla animantum,
Et quasi cursores vitas lampada tradunt.

Can we trace back this system to an origin, or do we discover only cycles of perpetual change, system following system, with ‘no trace of a beginning, no prospect of an end'? Perhaps the following considerations will incline the reader to adopt the opinion which ascribes a definite origin in time to life on the earth.

1. It is ascertained that, in passing downward through the lower Palæozoic Strata, the forms of life grow fewer and fewer, until in the lowest Cambrian Rocks they vanish entirely. In the thick series of these strata in the Longmynd (supposed to be 20000 ft. thick or more) hardly any traces of life occur. Yet these strata are of a kind such as might be expected to yield them; for they are not in general peroxidated, nor conglomeritic, nor much affected by metamorphic action, nor so much confused by

slaty cleavage as to render the search for fossils ineffectual by any of these circumstances. The materials are fine-grained or arenaceous, with or without mica, in laminæ and beds quite distinct and of various thickness, by no means unlikely to retain impressions of a delicate nature, such as those left by Graptolites, or Mollusks, or Annulose crawlers, Indeed, one or two such traces are supposed to have been recognized, so that the almost total absence of the traces of life in this enormous series is best understood by the supposition that in these parts of the sea little or no life existed. But the same remark of the excessive rarity of life in the lower deposits is made in North America, in Norway, and in Bohemia, countries well searched for this very purpose, so that all our observations lead to the conviction that the lowest of all the strata are quite deficient of organic remains. The absence is general—it appears due to a general cause. Is it not probable that during these very early periods the ocean and its sediments were nearly devoid of plants and animals, and in the earliest time of all which is represented by sediments, quite deprived of such ?

2. The variety of life in the sea continually augments from the lower Cambrian rocks upwards, in such a way as to leave no doubt of the richer fauna of the Llandeilo series being a part of the same

natural system as that of the Lingula flags below, and of the Caradoc beds above. Thus in tracing backward the series of Siluro-Cambrian life it appears to diminish, and disappear in a few forms, if not in a single form, before we reach the base of the strata. Thus we seem to have, as nearly as can be had, the evidence of the beginning of the earliest traceable life after the commencement of the earliest non-metamorphic strata.

3. In tracing downwards the various classes of marine animals, we find them to disappear separately at several successive zones of strata, so that for each class we have reason to think the origin in time is reached, or at least approached. Thus Fishes are not found below the Ludlow Rocks; Rhynchonella and Avicula hardly pass below Caradoc beds; Dimyaria and Cephalopoda grow very rare in the Llandeilo Rocks; and Lingula is almost the solitary occupant of the lowest laminæ of the fossiliferous beds of Wales.

No doubt it is open to any one to compare this approach to a Hypozoic Zero, with the reductions of life to a minimum above the Palæozoic and above the Mesozoic deposits; and to suppose that below the Palæozoics were other earlier strata and earlier systems of life, though they are now all lost in the general metamorphism which has produced the

Gneiss and Mica Schist. No one is likely to believe this however who attends seriously to the facts regarding the successive appearance of the classes, orders, families, genera and species, as we search the records of Geological time.

EARLIEST SYSTEMS OF LIFE.

The earliest system of life of which monuments remain is found in that very low part of the fossiliferous rocks of Wales, to which Professor Sedgwick first directed attention, now called the Lingula Zone. Perhaps the fossils of Bray, in the county of Wicklow, may belong to an older group of the strata, but this is assumed rather than proved. Of the two generic forms there discovered, Oldhamia may be a plant; Histioderma is an Annelid, probably of the Cephalobranchiate order. Taking our stand on the now well-explored Lingula Zone, but including in our Survey the forms just mentioned, we find the following characters of marine life, for no trace of terrestrial or of Fresh-water organization has been observed.

Zoophyta ?
Annelida

One genus. Oldhamia, 2 species.
2 genera.

Histioderma-Arenicolites

2 species.

[ocr errors]

Crustacea 7 genera. Agnostus--Conocephalus

Ellipsocephalus Hymenocaris — Olenus - Paradoxides - Protichniites

11 species. Polyzoa One genus. Dictyonema-1 species. Brachiopoda 2 genera. Lingula—Orthis. 3 species.

The list is taken from a valuable catalogue of the Lower Palæozoic Fossils of the British Isles, in Murchison's Siluria, Ed. 11. 1859? The Crustacea belong mostly to the great family of Trilobitidæ. Norway and North America yield each a series of the same general character. A larger number of species has been detected by M. Barrande in Bohemia, with the same general aspect. Individuals of the Oleni, Lingulæ and Oldhamiæ are abundant in certain layers, but a large portion of the strata of this 'Primordial' Zone is devoid of all traces of life.

Small as is this series of generic forms, we recognize representatives of each of the three great branches of the Invertebrata—there are carnivorous feeders as well as creatures nourished by cellular or vegetable food—and Fucoids are traced both in Norway and Malvern, in beds below the Oleni. In

1 Three species of Oleni found by myself in the black shales of Malvern are included in this zone, though in the Catalogue referred to they are placed in the Caradoc formation. Murchison, however, assigns these black shales to the early date here adopted for them. Siluria, 45 et seq.

« ПредыдущаяПродолжить »