PERSISTENT TYPES OF LIFE 393 



The regularity and evenness of the dentition of the 

 Anoplotherium, as contrasted with that of existing Artio- 

 dactyles, and the assumed nearer approach of the dentition 

 of certain ancient Carnivores to the typical arrangement, 

 have also been cited as exemplifications of a law of pro- 

 gressive development, but I know of no other cases based 

 on positive evidence which are worthy of particular notice. 



What then does an impartial survey of the positively 

 ascertained truths of paleontology testify in relation to 

 the common doctrines of progressive modification, which 

 suppose that modification to have taken place by a necessary 

 progress from more to less embryonic forms, or from more 

 to less generalized types, within the limits of the period 

 represented by the fossiliferous rocks ? 



It negatives those doctrines ; for it either shows us no 

 evidence of any such modification, or demonstrates it to 

 have been very slight ; and as to the nature of that modifica- 

 tion, it yields no evidence whatsoever that the earlier 

 members of any long-continued group were more generalized 

 in structure than the later ones. To a certain extent, 

 indeed, it may be said that imperfect ossification of the 

 vertebral column is an embryonic character ; but, on the 

 other hand, it would be extremely incorrect to suppose 

 that the vertebral columns of the older Vertebrata are in 

 any sense embryonic in their whole structure. 



Obviously, if the earliest fossiliferous rocks now known 

 are coeval with the commencement of life, and if their 

 contents give us any just conception of the nature and the 

 extent of the earliest fauna and flora, the insignificant 

 amount of modification which can be demonstrated to 

 have taken place in any one group of animals, or plants, 

 is quite incompatible with the hypothesis that all living 

 forms are the results of a necessary process of progressive 

 development, entirely comprised within the time represented 

 by the fossiliferous rocks. 



Contrariwise, any admissible hypothesis of progressive 

 modification must be compatible with persistence without 

 progression, through indefinite periods. And should such 

 an hypothesis eventually be proved to be true, in the only 

 way in which it can be demonstrated, viz. by observation 

 and experiment upon the existing forms of life, the con- 

 clusion will inevitably present itself, that the Paleozoic, 

 Mesozoic, and Cainozoic faunae and florae, taken together, 

 bear somewhat the same proportion to the whole series of 



