426 DAVIDSON ON FOSSIL BRACIIIOPODA. CHAP. xxn. 



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. 



Davidson on Fossil Brachiopoda. 



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 Eecent periods, 

 and still retain in the existing seas the identical shape and 



