SUCCESSIVE SYSTEMS OF MARINE INVERTEBRAL

LIFE.

The system of life thus constituted in the seas of the most ancient period so far resembles the system now established in modern oceans as to contain the same classes with similar functions and dependencies. But there are great differences in the relative proportions of the classes, and of the tribes which are included in them. And these differences are to a certain extent dependent on the elapsed time; classes at first very small have grown very large, others once predominant have been greatly diminished. To make this evident it will be useful to give up the mere enumeration of species in each class, and to adopt as a basis of representation the proportions in which the classes stand to each other in each period.

This can be easily done by equating the sum of the species in each period to 1000, and the number of species in each class to its proportion of the whole. Choosing for this purpose eight principal classes or assemblages, and tracing their relative proportions in successive periods, we arrive at the result represented in the parallelogram, Fig. 6.

In this representation of the relative proportions of the several classes in successive geological periods,

R. L.

G

the arrangement is purposely made to shew by the blue tint those classes which suffer diminution with time, and by the red tint those which from small beginnings grow to great preponderance, while the yellow tint is assigned to classes which scarcely appear in the early period, but swell out in the middle of the scale so as to equal or overmatch either of the other classes Thus appears in a striking light the great difference between the systems of oceanic life in earlier and later periods, the nature of this difference, and something of the method of variation which binds the whole into one plan, and connects the dawn of created life with this our breathing world.

In all the great periods the numerically prevalent life among the Invertebrata of the Sea appears in the Molluscous division1.

In the three grand periods the order of prevalence is thus found.

Cænozoic Period ... Gasteropoda, Dimyaria, Monomyaria, Echinodermata, Zoophyta, Crustacea,

Cephalopoda, Brachiopoda.

Mesozoic Period ... Dimyaria, Cephalopoda, Gasteropoda, Monomyaria, Echinodermata, Brachio

poda, Zoophyta, Crustacea.

Palæozoic Period... Brachiopoda, Gasteropoda, Zoophyta, Dimyaria, Cephalopoda, Echinodermata, Crustacea, Monomyaria.

Taken in the order of their total numerical su

periority, the classes range thus:

Cænozoic. Mesozoic. Palæozoic.

Total.

The sign + is placed to the maximum number in each class, shewing that the maximum is attained in the

Cænozoic Period, by Gasteropoda.

Mesozoic Period, by Dimyaria, Cephalopoda, Monomyaria,

Echinodermata.

Palæozoic Period, by Brachiopoda, Zoophyta, Crustacea.

These results afford but slight encouragement to the speculation of the inferiority of the earlier and superiority of the later systems, and of continual progress upward in the organization of animals. In proportion to the elapsed time the changes make progress, but these changes are not always in the sense of uninterrupted advance from inferior to superior forms.

G2

For example, Cephalopoda, by universal consent, stand at the head of the Molluscous kingdom of animals; but their origin is of the same date as that of the Mollusca generally: they rapidly rise to importance, but pass the maximum in the Mesozoic period, and are now but a small and scattered part of the inhabitants of the sea, enormously outnumbered by the inferior races of Gasteropoda and Dimyaria. Thus, starting from an equal basis, the superior class has lost in the 'struggle for existence.' But we must examine this subject on other occasions, after gathering additional data.

CHANGES IN MARINE ANIMALS WITH ELAPSED
TIME.

The principal classes of marine fossil Invertebrata have now been traced from what seems to be their origin, to or beyond the epoch of their greatest prevalence. We have, in fact, taken the census of our marine inhabitants at several periods. It remains to examine them with reference to their structure and grade of organization in these periods; to compare, for instance, the Crustacea and Mollusca of one period with those of another, and thus to learn the amount of variation in this respect from period to period, and what is the method of variation. We may include in this inquiry Fishes, which to the extent of 736 species have been recorded in the British strata, Marine Reptiles, which are less numerous, and Cetacea, which are rare.

Amorphozoa offer in this respect little for remark. Belonging to the lowest grade of animal organization-by some naturalists of eminence counted among plants-Sponges are nowhere very abundant except in the Cretaceous Strata, where some forms occur much like existing tribes, and similarly furnished with siliceous spicula, and in some cases with a network of anastomozing fibres.

Foraminifera. These minute cellular structures occur perhaps in most of the limestones and clays, but at present the greater number are quoted from the Upper Mesozoic and Cænozoic Strata. In general they correspond much and even remarkably to existing kinds. Some fossil groups, extremely variable in form, appear quite undistinguishable from recent examples; so that by this tribe of animals there appears a continuity of some specific forms from Mesozoic through Cænozoic to recent times1.

Zoophyta. The fossil groups are principally of the kinds which secrete the stony support known as Coral, and belong to the division of Zoantharia. With hardly an exception (Gorgonia?) the numerous