INTRODUCTION. XXI 



become stouter ; and when the endoskeletal base of the fin has 

 been reduced to one small row of elements, these never multiply 

 again even the lobe of the great pectoral fin of the modern 

 angler-fish (Lophius) is formed solely by the elongation of two 

 of these little elements. The same phenomenon is observed 

 among mammals ; the number of digits may be reduced even 

 to one, but when any reduction has taken place the original 

 pentadactylism is never restored. Finally, in the case of all 

 vertebrates, the teeth tend to degenerate ; first the supply of 

 successional teeth is stopped, then the one "permanent" set 

 disappears and when either of these phases of degeneration is 

 accomplished, the original state is never recovered. 



Specialization. It is thus evident that among animals there 

 are certain definite and irreversible lines of progression, and 

 other equally definite and irreversible lines of degeneration. 

 At the same time the Palaeontology of the Vertebrata shows 

 most clearly that, on the whole, the evolution of these organisms 

 has proceeded from the general to the special, while in every 

 successive period of the earth's history some group has risen to 

 a higher position in the zoological scale than any previously 

 attained. This phenomenon has already been mentioned in 

 recounting the order of succession of the various classes (p. xvii). 



Expression Points. All known facts appear to suggest 

 that the processes of evolution have not operated in a gradual 

 and uniform manner, but that there has been a certain amount 

 of rhythm in the course. A dominant old race at the beginning 

 of its greatest vigour seems to give origin to a new type showing 

 some fundamental change ; this advanced form then seems to 

 be driven from all the areas where the dominant ancestral race 

 reigns supreme and evolution in the latter becomes com- 

 paratively insignificant. Meanwhile the banished type has 

 acquired great developmental energy, and finally it spreads over 

 every habitable region, replacing the effete race which originally 

 produced it. Another " expression point " (to use Cope's apt 

 term) is thus reached, and the phenomenon is repeated. The 

 actinopterygian fishes furnish an interesting illustration. The 

 earliest known member of this order (Gheirolepis) appears as an 

 insignificant item in the Lower Devonian fauna, where crosso- 



