PENGELLT — FOSSILS OF DETOTf AND COEN^WALL. 



25 



greatest number of species in Silurian, two in Devonian, and twenty- 

 three in Carboniferous times ; thus giving a total of thirty-eight, and, 

 consequently, leaving three genera which had not a maximum specific 

 development in any one period. 



It appears, then, that the genera found in the Devonian era, as 

 represented in Devon and Cornwall, even when those peculiar to it 

 are included, yield a less aggregate number of species, that the ave- 

 rage number of species per genus is smaller, and that the genera 

 having tlieir maximum specific development are fewer in Devonian 

 than in either Silurian or Carboniferous times, and that in each of 

 these particulars the Carboniferous surpasses the Silurian age. 



Such appear to be the prominent facts in connexion with the sub- 

 ject immediately before us. What is their interpretation ? This is 

 a problem more easily proposed tlian solved. Are we to believe that 

 our knowledge of the geological record is too imperfect to warrant 

 any important generalizations ? Do our museums fully represent the 

 fossilized remains of bj^gone forms of life ? Are all the extinct organ- 

 isms which have been exhumed registered in the published lists ? Is 

 the record itself, inscribed on rocky tablets, so incomplete as to be 

 altogether incapable of revealing to us the physical and organic his- 

 tory of our planet ? Are the notions of biologists respecting specific 

 distinctions, whatever they may be, sufficiently mature and uniform 

 to warrant our relying on them ? Something must doubtless be con- 

 ceded on each of these points, but still there cannot but be a large 

 outstanding quantity of fact incapable of being thus explained away.- 

 The problem demands some other solution. 



Suppose it true that in some cases the organic dissimilarity which 

 has been described was due to a difference in the mineral character 

 of the ancient sea-bottom, such as was mentioned in the case of 

 Lower South Devon and Lower Cornwall ; still, when we have two 

 areas, like Lower South and Lower North Devon, consisting of con- 

 temporary, almost contiguous, and scarcely dissimilar deposits, one 

 rich and the other poor in the variety of its organic remains, having 

 together two hundred and four species with no more than eight in 

 common, some other solution is obviously required. AVas there a 

 terrestrial barrier separating the two areas ? Was the central dis- 

 trict occupied by dry land, stretching far both east and west, while 

 the waves of the Devonian ocean rolled over the north and south of 

 the county ? for it need not be stated that the deposits we are con- 

 sidering are eminently marine. It may be too much to answer this 

 question with an unqualified negative ; it is easier to determine, at 

 least, some of the ancient oceanic areas than to say where lay the 

 contemporary continents and islands. Nevertheless, the rocks now 

 separating the areas in question, namely, the granites, the carboni- 

 ferous beds, and the red conglomerates (or, more correctly, breccias), 

 are unquestionably more modern than those now under notice ; nor 

 is the structure of the latter such as to imply the immediate proxi- 

 mity of dry land in that quarter. 



Besides, eight species actually did migrate from one area to the 



VOL. Y. * E 



