790 ANATOMY OF VERTEBRATES. 



The progress of Palaeontology since 1830 has brought to light 

 many missing links unknown to the founder of the science. My 

 own share in the labour led me, after a few years' research, to 

 discern what I believed, and still hold, to be a tendency to a more 

 generalised, or less specialised, organisation as species recede in 

 date of existence from the present time, 1 Even instances which 

 to some have appeared to oppose the rule, really exemplify it. 

 The little marsupial carnivore, e. g., of the Purbeck beds, Pla- 

 giaulax (p. 294, fig. 234), retained the typical numbers of premo- 

 lars (p. 1-4), all of them being carnassials : the more modified 

 pliocene Thylacoleo had them reduced to the last (p. 4, fig. 233). 

 So likewise in the later placental Carnivora, the eocene form 

 Hycenodon, fig. 266, had the typical number of teeth, the three 

 true molars here showino- the carnassial form : in the existing 



o 



Hya3na and Felines the carnassials are reduced to, or concentrated 

 in, a single molar. The oolitic Phascolotherium, with the typical 

 marsupial number of teeth, shows less differentiation in their form 

 than in modern Opossums and Dasyures : the oolitic Amphitheria 

 and Pal&otheria manifest an earlier and more generalised type 

 of dentition in the great number and similarity of character of 

 their small molars. Both Anoplotherium and Palceotherium, with 

 the majority of eocene placental Mammals, had the type-dentition 

 of diphyodonts. 2 



The two notable examples of Cuvier's powers of restoration, 

 viewed as Pachyderms, must have seemed widely different from 

 any of the existing species of the order, and were so deemed. 

 The Anoplotherium more especially, among its singular peculiari- 

 ties, unexpectedly exemplified one dental character, previously 

 known only in the human subject. These seeming anomalies, 

 however, lost much of their import as evidence of insulated form, 

 or special creation, when they came to be viewed by the light of 

 the law of the ' more generalised character of extinct species.' 

 Such law in its application to Anoplotherium also exemplifies the 

 analogy between the earlier species of a class and the earlier 

 stages of a fostus. When, for example, the divided metapodials, 

 the persistent upper incisors, and the hornless cranium of the 

 Anoplothere were recognised as retentions of ' foetal peculiarities ' 



1 CCXLIX. Ed. 1843, pp. 129, 165; Ed. 1855, pp. 223, 332, 342. CLXXX. and xvii'. 

 pp. 1, 361, passim. Agassiz had been struck by indications of the same law in fossil 

 iishes, and expressed it by the analogy of foetal andmature structures (cccxxix". (1844) 

 p. xxvi.), and this, in some degree, is true. The earlier forms of Mammalia, however, 

 are not toothless, have rather an excess of teeth as compared with later and modern 

 forms ; "but they exemplify, in the main, a more ' generalised ' type. 



2 v. p. 524. CLXXX. p. 361. 



