Page:Geological Evidences of the Antiquity of Man.djvu/446

426 then endeavour to imagine the number of forms of the genus Rubus which may now exist, or probably have existed in Europe, and in regions intervening between Europe and Australia, comprehending all which may have flourished in tertiary and post-tertiary periods, we shall perceive how little stress should be laid on arguments founded on the assumed absence of missing links in the flora as it now exists.

If in the battle of life the competition is keenest between closely allied varieties and species, as Mr. Darwin contends, many forms can never be of long duration, nor have a wide range, and these must often pass away without leaving behind them any fossil memorials. In this manner we may account for many breaks in the series which no future researches will ever fill up.

It is from fossil conchology more than from any other department of the organic world that we may hope to derive traces of a transition from certain types to others, and fossil memorials of all the intermediate shades of form. We may especially hope to gain this information from the study of some of the lower groups, such as the Brachiopoda, which are persistent in type, so that the thread of our enquiry is less likely to be interrupted by breaks in the sequence of the fossiliferous rocks. The splendid monograph just concluded by Mr. Davidson, on the British Brachiopoda, illustrates, in the first place, the tendency of certain generic forms in this division of the mollusca to be persistent throughout the whole range of geological time yet known to us; for the four genera Rhynconella, Crania, Discina, and Lingula have been traced through the Silurian, Devonian, Carboniferous, Permian, Jurassic, Cretaceous, Tertiary, and Recent periods, and still retain in the existing seas the identical shape and