190 GEOLOGICAL DISTRIBUTION*. 



existence to develop after an interval of millions of years ? This 

 seems very improbable, but yet it may be so ; but why may it not 

 also have been that, in the Carboniferous form, we had prema- 

 ture evolution, with subsequent extinction, and that at a much 

 more recent period a re evolution of the same form, under more 

 favourable circumstances, took place ? The case is certainly very 

 extraordinary, and probably has no parallel in the history of pale- 

 ontology. 



On Appearance and Extinction.— It would naturally be sup- 

 posed on the hyiDothesis of evolution that the introduction of all 

 species must be a very gradual one, for it can scarcely be conceived 

 that the laws governing the formation of new species through de- 

 scent and modification could be anything but very slow in their 

 action. So true is this, that Darwin has himself admitted, that 

 ' ' if numerous species, belonging to the same genera or families, 

 have really started into life at once, the fact would be fatal to 

 the theory of descent, with slow modification through natural se- 

 lection " (" Origin of Species"). Yet if we glance over the geo- 

 logical record we cannot fail to note a very considerable num- 

 ber of seemingly flagrant contradictions, groups of allied species 

 appearing in almost every formation with apparently the greatest 

 possible abruptness. We have but to instance as examples the 

 genera of articulate brachiopods, and the tabulate corals of the 

 Silurian period, the ganoid fishes of the Devonian period, the 

 Tertiary jjlacental mammals, and the foraminiferal genus Num- 

 mulites, already referred to. Probably there exists no more strik- 

 ing illustration of the abrupt or sudden development of a family 

 than is furnished by the Nautilidae. a group of animals for whose 

 elucidation we are principally indebted to the labours of M. Bar- 

 rande. Of this family of cephalopods, which comprises probably 

 upwards of two thousand distinct species, no less than four hun- 

 dred and sixty-three species, referable to some fifteen or more 

 genera, are already represented in the Lower Silurian formation, 

 although from the preceding Cambrian deposits at best only two 

 well-authenticated forms (Cyrtoceras praecox and Orthoceras seri- 

 ceum) are known. Of this number about two hundred and sixty 

 belong to the genus Orthoceras itself, ninety to CjTtoceras, and 

 forty-six to Endoceras, the last, a genus restricted absolutely to 

 the Lower Silurian deposits. Facts such as these have been eager- 



