Page:The Geologist, volume 5.djvu/39

Rh 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 average number of species per genus is smaller, and that the genera having their 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 subject immediately before us. What is their interpretation? This is a problem more easily proposed than 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 bygone forms of life? Are all the extinct organisms 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 history 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 conceded 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. Was there a terrestrial barrier separating the two areas? Was the central district 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 considering 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 carboniferous 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 proximity of dry land in that quarter.

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