370 Revieics — A. Smith WoodimrcVs Vertebrate Palceontology. 



"Definite Direction of Involution. — Under such circumstances it can 

 be readily understood, that the time has not yet arrived for deducing 

 general laws from the data of Palaeontology. In fact, our knowledge 

 of the evolution even of the Yertebrata is most casual and fragmentary 

 in character. Nevertheless, sufficient is known to indicate that changes 

 in the vertebrate skeleton have taken place in a certain definite and 

 irreversible order ; and the relative age of two skeletons of the same 

 type of animal of widely different periods can readily be determined 

 at a glance by an expert. For example, among the eai'ly Palaeozoic 

 fishes there are many heavily-armoured forms with very little 

 ossification in the endoskeleton and only incipient vertebral centra. 

 The endoskeleton does not begin to ossify to any noteworthy 

 extent until the exoskeleton atrophies ; eventually at a later period 

 the bony endoskeleton is the all-impol-tant part of the framework. 

 Now, it so happens that in certain Mesozoic Pycnodont fishes 

 (Mesturus) and the Tertiar^'^ coffer-fishes (OstraciontidEe) the rigid 

 dermal armour is again acquired ; but it is apparently contrary to 

 law for the endoskeleton to revert to its primitive state, as observed 

 in the Palaeozoic fishes just mentioned; the parts of the axial 

 skeleton simply become rigidly fixed, and are nearly as well ossified 

 as in their unarmoured contemporaries and relatives. When the 

 vertebral centra have once become fully formed, indeed, they never 

 degenerate to allow the unconstricted notochord to persist again. To 

 take another example, the lobate fins with endoskeletal supports in 

 the earliest fishes always appear to tend towards atrophy, while 

 the dermal rays surrounding them 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 (Lophins) 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 Paleontology 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. 



" 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 



