xVili INTRODUCTION. 
as applied to the subdivisions of the Palaeozoic rocks—such as Carboniferous, Devonian, 
Silurian, &c.; and since then I have repeatedly used the word system in this extended 
sense, and recommended its adoption®. Strictly speaking, all organic types, of whatever date, 
belong to one great Systema Nature. ad this not been true, paleontology could never 
have risen into a science. Still there is such a great organic interval between the /lora 
and Fauna of the Primary and Secondary groups, that we may conveniently, and without 
any risk of error, describe each of them as belonging to a separate organic system. A single 
glance convinces us that the /awna of the Secondary period greatly differs from that of the 
Primary or Palzozoic. But the case is widely different in comparing the organic types of 
the subdivisions of the Paleozoic rocks. Both physically and palzontologically, we find a 
great difficulty in drawing fixed lines between these subdivisions; nor at this moment are 
geologists in all cases agreed as to the true places where such lines should be drawn. 
Not only have the Palsozoic rocks a peculiar organic type, but there are several 
species which run through nearly all the subordinate groups; and seem, therefore, to unite 
them into one Paleozoic System. For example, Mavosites alveolaris, and Ff. Gothlandica, 
Stenopora fibrosa, Spirigerina reticularis, and Leptagonia depressa are all found in the Bala 
group; yet three of them rise into the Devonian series, and the remaining two rise into the 
Carboniferous. And this list of common species might, I doubt not, be considerably increased. 
There is, then, no incongruity, and certainly no risk of error, in applying the term System to 
all the Paleozoic rocks; while there is some incongruity, and (as we know from experience) 
a great risk of error, in applying the definite term System to the great subdivisions of the 
Paleeozic rocks, such as the Carboniferous, Devonian, and Silurian. The use of the word 
Series, in describing the subdivisions of the Paleozoic System, is by no means an innovation; 
but is, on the contrary, a return to the language formerly in very common use among the 
members of the Geological Society; as any one may see on turning over the pages of their 
early Volumes. 
Among the most exact continental writers, the word Terrain is used for collective, and 
sometimes imperfectly-defined, groups of rocks; while the word Systeme is applied to the 
subordinate and well-defined groups, whether natural or artificial. This sense of the word 
system is consistent and philosophical, and can seldom lead to mistakes of interpretation. In 
England we have no technical word corresponding to Terrain, though the word Country might 
perhaps be used to supply the want; and the word System has frequently been used by us 
in a loose and ill-defined sense; sometimes being applied to certain well-defined groups, on 
evidence partly physical and partly paleontological; sometimes to utterly undefined groups, 
on evidence purely paleontological. And hence has been introduced an incongruous nomen- 
clature, which is sometimes geographical, sometimes paleontological, and sometimes neither 
geographical nor paleontological. By adopting a definite nomenclature, before we are well 
acquainted with the physical and palzontological phenomena on which it must ultimately 
* For example, Proceedings of the Geological Society, Vol. 1V. June, 1843, p, 221. 
