XXll INTRODUCTION. 



pterygian and dipnoan fishes are dominant. When the latter 

 begin to decline in the Lower Carboniferous, the sub-order to 

 which Cheirolepis belongs (Chondrostei) suddenly appears in 

 overwhelming variety. By the period of the Upper Permian 

 another fundamental advance has taken place the Proto- 

 spondyli have arisen; but only a solitary genus is observed 

 among the hosts of the dominant race. In the Trias the new 

 type becomes supreme, and at the same time the next higher 

 sub-order, that of the Isospondyli, begins to appear. This 

 lingers on in the midst of the dominant Protospondyli during 

 the Jurassic period, and then in the Cretaceous this and still 

 higher sub-orders suddenly replace the earlier types and 

 inaugurate a race which has subsequently changed only to an 

 insignificant extent. The Mammalia afford another illustration 

 of the same phenomenon. The reptilian class shows the closest 

 approximation to that of the Mammalia at the dawn of the 

 Mesozoic epoch, when it is just beginning to replace the older 

 Batrachia. In rocks of this age on all four continents Europe, 

 Asia, Africa, and America there are numerous remains of the 

 mammal-like Anomodontia (as they are termed, p. 144). After- 

 wards, not a trace of these " missing links " is known ; and, 

 with the exception of the insignificant small jaws of possible 

 Prototheria and Metatheria in the Jurassic and Cretaceous of 

 England and North America, mammals do not appear either in 

 Europe, Africa, or North and South America until the base of 

 the Eocene, when they suddenly become dominant and are 

 already differentiated. 



Parallelism in Evolution. As nothing is yet known of the 

 supposed refuges to which the incipient new races have betaken 

 themselves to differentiate and acquire vital energy, we can 

 merely assume them as a tentative hypothesis. But even when 

 the facts are abundantly manifest, it is often difficult to settle 

 the most elementary questions by direct reference to them. 

 The problem of parallelism in evolution is one of these. It is 

 necessary to determine whether the same animal can be evolved 

 simultaneously in more than one region from distinct series of 

 ancestors. Are the pumas and jaguars of America, for example, 

 wandering cousins of the lions and leopards of the Old World, 



