372 Remeivs — A. Smith WoodtcartVs Vertebrate Pal(Bontology. 



closely similar to, but not quite identical with, the ancestral series 

 found in the same order in the corresponding rocks of North 

 America. Here the facts are tolerably well known, but they admit 

 of more than one interpretation. An easy land-connection between 

 Europe and North America, throughout the Tertiary period, if 

 allowed by geological considerations, might account for the 

 phenomena observed, even if all the horse-like animals were 

 evolved fx'oin one stock in one and the same area. 



" Theory of Recapitulation. — There is also the well-known and 

 widely-accepted principle that the stages in the development of an 

 individual organism at the present day repeat in a general manner 

 the successive phases through which the ancestors of that organism 

 have passed in former periods of the earth's history. There is no 

 doubt, for example, that in the course of its individual development 

 the homocercal tail of a modern bony fish passes through the same 

 stages as those successively exhibited by the majority of the adult 

 fishes at the different geological epochs. It is also evident that the 

 family of deer (Cervidee) has gradually acquired complex antlers in 

 precisely the same manner as every modern stag acquires them 

 during the course of its individual life. Again, the ' cloven foot ' 

 of the existing ruminant appears in the embryo with separated 

 metapodial bones, like those of the adult ancestral ruminants. It is 

 also tolerably certain (though fossils have not yet provided absolute 

 demonstration) that the rudimentary teeth and hind limbs of 

 the existing whalebone whales (Mj'stacoceti) are inherited from 

 functionally toothed quadrupedal ancestors. Embryology, however, 

 cannot afford much precise information concerning these processes 

 of evolution, because an embryo usually exists under physiological 

 conditions totally distinct from those influencing an adult. The 

 embryo exhibits features derived from its ancestors {palingenetic 

 characters), inextricably mingled with features due to the peculiar 

 circumstances under which it develops {ccenogenetic characters). In 

 most cases it has hitherto been impossible to distinguish these two 

 sets of characters satisfactorily ; and a final appeal must thus be 

 made to Palasontology, notwithstanding its imperfections, to 

 determine the laws by which evolution proceeds. 



" Origin of the Vertehrata. — Perhaps the most disappointing element 

 in palffiontological results thus far, is the lack of all information 

 concerning the origin of the great sub-kingdoms or phyla of animals. 

 Even in what might ajDpear to be the most promising case, namely, 

 that of the Vertehrata, there are no known facts distinctly favouring 

 any of the rival theories concerning their origin based on embryology. 

 Possibly all the earliest types were destitute of hard parts, and thus 

 incapable of fossilization. In any case, the oldest Ostracoderms 

 (p. 3) from the Upper Silurian and Lower Devonian, sometimes 

 claimed as the immediate allies of the Crustacean or Arachnid 

 Merostomata of the same period, are fundamentally different from 

 the latter in every character which admits of detailed comparison ; 

 they are to be regarded merely as an interesting example of mimetic 

 resemblance between organisms of two different grades, adapted to 

 live in the same way and under precisely similar conditions." 



