58 



former times. A consideration of such instances of modern partial 

 or total extinctions may best throw light on, and suggest the truest 

 notions of, the causes of ancient extinctions. 



As to the successions, or coming in, of new species, one might 

 speculate on the. gradual modifiability of the individual; on the 

 tendency of certain varieties to survive local changes, and thus 

 progressively diverge from an older type ; on the production and 

 fertility of monstrous offspring; on the possibility, e.g. of a variety 

 of auk being occasionally hatched with a somewhat longer winglet, 

 and a dwarfed stature ; on the probability of such a variety better 

 adapting itself to the changing climate or other conditions than 

 the old type of such an origin of Alca torda, e. g. ; but to what 

 purpose ? Past experience of the chance aims of human fancy, 

 unchecked and unguided by observed facts, shews how widely they 

 have ever glanced away from the gold centre of truth. 



The sum of the evidence which has been obtained appears to 

 prove that the successive extinction of Amphitheria, Spalacotheria, 

 Triconodons, and other mesozoic forms of mammals, has been 

 followed by the introduction of much more numerous, varied, and 

 higher-organised forms of the class, during the tertiary periods. 



There are, however, geologists who maintain that this is an 

 assumption, based upon a partial knowledge of the facts. Mere 

 negative evidence, they allege, can never satisfactorily establish 

 the proposition that the mammalian class is of late introduction, 

 nor prevent the conjecture that it may have been as richly repre- 

 sented in secondary as in tertiary times, could we but get evidence 

 of the terrestrial fauna of the oolitic continent. To this objection 

 I have to reply : in the palaeozoic strata, which, from their extent 

 and depth, indicate, in the earth's existence as a seat of organic 

 life, a period as prolonged as that which has followed their depo- 

 sition, no trace of mammals has been observed. It may be con- 

 ceded that, were mammals peculiar to dry land, such negative 

 evidence would weigh little in producing conviction of their non- 

 existence during the Silurian and Devonian seons, because the ex- 

 plored parts of such strata have been deposited from an ocean, and 

 the chance of finding a 'terrestrial and air-breathing creature's re- 

 mains in oceanic deposits is very remote. But, in the present 

 state of the warm-blooded, air-breathing, viviparous class, no genera 

 and species are represented by such numerous and widely dispersed 

 individuals, as those of the order Cetacea, which, under the guise 

 of fishes, dwell, and can only live, in the ocean. 



