II THE ORIGIN OF SPECIES 55 



clearly out of geological investigation than another, 

 it is, that the vast series of extinct animals and 

 plants is not divisible, as it was once supposed to 

 be, into distinct groups, separated by sharply- 

 marked boundaries. There are no great gulfs 

 between epochs and formations no successive 

 periods marked by the appearance of plants, of 

 water animals, and of land animals, en masse. 

 Every year adds to the list of links between 

 what the older geologists supposed to be widely 

 separated epochs : witness the crags linking the 

 drift with older tertiaries ; the Maestricht beds 

 linking the tertiaries with the chalk; the St. 

 Cassian beds exhibiting an abundant fauna of 

 mixed mesozoic and palaeozoic types, in rocks of an 

 epoch once supposed to be eminently poor in life ; 

 witness, lastly, the incessant disputes as to whether 

 a given stratum shall be reckoned devonian or 

 carboniferous, silurian or devonian, cambrian or 

 silurian. 



This truth is further illustrated in a most 

 interesting manner by the impartial and highly 

 competent testimony of M. Pictet, from whose 

 calculations of what percentage of the genera of 

 animals, existing in any formation, lived during 

 the preceding formation, it results that in no case 

 is the proportion less than one-third, or 33 per 

 cent. It is the triassic formation, or the com- 

 mencement of the mesozoic epoch, which has 

 received the smallest inheritance from preceding 



33 



