a THE ORIGIN OF SPECIES 55 
- elearly 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 paleozoic 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 
