GENERAL PICTURE OF PALAEOZOIC TIMES. 407 



lies, etc. According to Prof. Hall, there have been in the Silurian and 

 Devonian ages alone at least thirty almost complete changes of species. 

 The changes of genera are, of course, much less numerous, and those 

 of families still less than those of genera. These general laws may be 

 illustrated by any Palaeozoic order ; but I select the order of Trilobites, 

 because they are very numerous, very diversified, and well studied, and 

 because they came in with the Palaeozoic, continued throughout the 

 whole era, and then passed away forever. 



The diagram (Fig. 586) illustrates these laws in the order of Trilo- 

 bites. It is seen that this order continues through the whole era, 

 commencing in small numbers, reaching its highest development in 

 the Lower Silurian, and declining to the end. But the families are 

 changed several times. Six groups are given, to show how they come 

 and go successively. If we should attempt the distribution of genera, 

 the changes would be much more numerous, and of species still more so. 

 In the lower portion of the diagram we have attempted to show in a 

 very general way how the distribution of species of Calymene and 

 Acidaspis might be represented. 



General Comparison of the Fauna of Palaeozoic with that of Neozoic 

 Times. — The changes above explained w r ere gradual ; but at the end of 

 the Palaeozoic there occurred a more rapid and revolutionary change, 

 and the greatest which has ever occurred in the history of the organic 

 kingdom. As human history is primarily divided into Ancient and 

 Modern, so the whole history of the earth may be properly divided into 

 Palaeozoic and Neozoic times. We wish to contrast broadly the faunae 

 of these two great divisions of time. In the diagram on next page, 

 the vertical line represents the dividing line between the old and the 

 new time- world. In this country it is appropriately called the Appa- 

 lachian revolution. On the left is the Palaeozoic, on the right the 

 Neozoic. When families or orders of animals are placed on one or the 

 other side without mark, it means that they are the only kind of the 

 contrasted families found on that side, or nearly so. If the orders or 

 families so placed are marked with the sign -{-, it means that they are 

 the predominant hinds. For example, among Cephalopods, the Tetra- 

 branchs, or shelled family, are the only kinds found in the Palaeozoic ; 

 in the Neozoic, both families exist, but the Dibranchs or naked ones 

 vastly predominate. 



General Picture of Palaeozoic Times. 



Perhaps it is not inappropriate to group some of the more impor- 

 tant facts in a very brief outline-picture of Palaeozoic times. We must 

 imagine, then, wide seas and loiv continents of small extent; a hot, 

 moist, still air, loaded with carbonic acid, stifling and unsuited for the 

 life of warm-blooded animals. If an observer had walked along those 



