In this diagram the Passage Beds are taken at 500 ft. with 13 species, Ludlow 2000 ft. with 228 species, Wenlock 2500 ft. with 322 species, Llandovery 2500 ft. with 196 species, Caradoc 6800 ft. with 335 species, Llandeilo 6800 ft. with 104 species, Lingula beds 4000 ft. with 14 species. The area of the spaces corresponds to the number of species, thus shewing at a glance the relative richness in species of the several groups for equal thicknesses. This richness (expressed by numbers on the right side) rises from almost zero in the Lingula beds to a maximum in the upper part of the Wenlock series, and dies out almost to zero in the beds of Passage to the Old Red Sandstone system above.

What is here said of the Siluro-Cambrian or Lower Paleozoic Strata may be repeated with equal truth in reference to each of the systems of associated deposits: for in each the characteristic and prevalent fauna begins at a minimum, rises to a maximum, and dies away to a final minimum, to be followed by another system having similar phases. The most remarkable and prevalent of these surfaces or zones of least life are those two which separate the Palæozoic from the Mesozoic, and these from the Cenozoic Series. The Palæozoic Series dies out through the Permian system, and the Mesozoic rises slowly in the Trias; so the Mesozoic Series dies away in the uppermost

beds of the Cretaceous system, and the Cenozoic Series grows slowly through the lowest Eocene to a maximum in the later tertiaries.

A depression in the maximum occurs in the Palæozoic Series of Britain corresponding to the Devonian period, here so largely represented by peroxidated sediments; another in the Mesozoic period, in the uppermost Oolites, which are poor in comparison with the richer series of Oxford and Bath. These peculiarities are represented in the following diagram by a continuous curve, which corresponds to the numerical prevalence of life, and represents its rise and fall. Fig. 4.

R. L.

F

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 vitaï 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 Paleozoic 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