Изображения страниц

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 division'.

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, Brachiopoda, Zoophyta, Crustacea.


Only in the very earliest zone, Crustacea appear to predominate over Brachiopoda.

Palæozoic Period ... Brachiopoda, Gasteropoda, Zoophyta,

Dimyaria, Cephalopoda, Echinodermata,

Crustacea, Monomyaria. Taken in the order of their total numerical superiority, the classes range thus:

Cænozoic. Mesozoic. Palæozoic. Total, Gasteropoda.........662+ 389 401 1452 Dimyaria..... .394

499 +

342 1235 Brachiopoda 8 165 632+

805 Cephalopoda 12


336 744
Monomyaria 63 308+ 196 567
Echinodermata 41 245 + 225 511
Zoophyta....... 27 103


509 Crustacea

15 65



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

Canozoic Period, by Gasteropoda.
Mesozoic Period, by Dimyaria, Cephalopoda, Monomyaria,

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.


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.



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 times'.

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 genera of the Palæozoic systems belong to this division; the same is true for the Mesozoic series; and

1 Carpenter, Proc. of Roy. Soc. 1855—60. Jones and Parker, Journal of the Geological Society of London, 1860.

; even in the Cænozoic strata Alcyonoid Zoophyta are rare. Here, then, is one great order of Zoophyta continuous through the whole series. The genera change with the successive deposits, but there is one remarkable law of structure which is characteristic of period. The radiating plates of the Coral are in a young state pretty regular in number and in the mode of division. In all the Palæozoic Strata the primary or principal plates are four, or some multiple of four; in all the Mesozoic and Cænozoic Strata they are six, or some multiple of six. This striking generalization, due to Milne Edwards and the late Jules Haime, is thought to be subject to no more than solitary exceptions. It suggests the reflection that the persistence of characters which we observe in modern living nature was quite as remarkable in ancient organizations, and throws a heavy weight into the scale against the doctrine of the later forms of life being derived from earlier types, through natural variations integrated by time.

Echinodermata.—Six fossil groups represent this beautiful class of animals, viz. Crinoidea, Blastoidea, Cystoidea, Ophiuroidea, Asteroidea, Echinoidea. Of these, two are only known fossil (Blastoidea and Cystoidea), and they belong to the Palæozoic Strata.

« ПредыдущаяПродолжить »