112 INVERTEBRATE PALAEONTOLOGY 



reached a prominent acme in the Devonian. Of the 

 nine families, no less than seven were differentiated in the 

 Ordovician and Silurian periods. But the pre-Devonian 

 genera number only twenty-four, whereas twenty-eight 

 originated in the Devonian period alone ; specific analysis 

 would bring out yet more strongly the acmaic quality of 

 minor differentiation at that time. Only twelve genera 

 arose in the Carboniferous, and five in the Permian ; but 

 in the Trias, when the superfamily was approaching ex- 

 tinction, no less than twenty-one new genera appeared. 

 In spite of this gerontic effort in specialization, the whole 

 group disappeared before the end of the Liassic stage. 



In addition to the differential rates of evolution indi- 

 cated above (which are common to all, or almost all, 

 series) there is a type of irregularity which, while as yet 

 unrecognized in many groups, seems to modify the early 

 phases of racial-life in several cases. It may be de- 

 scribed as " deferred specialization." The outstanding 

 example of this early retardation is to be found in the 

 Mammalia. Although the Mammalian stock arose in 

 Rhaetic, or perhaps Permo-Triassic, times, its members 

 showed little progress throughout the Mesozoic era. At 

 the commencement of the Cainozoic, extraordinary 

 acceleration, typically " neanic " in character and pro- 

 ducts, rapidly brought the class into a position of faunal 

 predominance. It is possible to argue, in this case, that 

 the importance of Mesozoic Reptilia was such that there 

 was no room for a second powerful group of Vertebrates ; 

 and it is true that the rise of Mammalia was deferred 

 until the Reptilia were reduced to insignificant propor- 

 tions in the fauna. But such an argument would in no 

 way account for the slow specialization of the Mammalia. 

 It might have been expected that the two great classes 

 would have competed for mastery, the Mammals dissi- 

 pating their racial energy in vain endeavours to make 



