Chap. X. 
GEOLOGICAL SUCCESSION. 
313 
nor disappearance of their many now extinct species has 
been simultaneous in each separate formation. 
Species of different genera and classes have not 
changed at the same rate, or in the same degree. In 
the oldest tertiary beds a few living shells may still be 
found in the midst of a multitude of extinct forms. 
Falconer has given a striking instance of a similar fact, 
in an existing crocodile associated with many strange 
and lost mammals and reptiles in the sub-Himalayan 
deposits. The Silurian Lingula differs but little from 
the living species of this genus; whereas most of the 
other Silurian Molluscs and all the Crustaceans have 
changed greatly. The productions of the land seem to 
change at a quicker rate than those of the sea, of which 
a striking instance has lately been observed in Switzer¬ 
land. There is some reason to believe that organisms, 
considered high in the scale of nature, change more 
quickly than those that are low: though there are ex¬ 
ceptions to this rule. The amount of organic change, 
as Pictet has remarked, does not strictly correspond 
with the succession of our geological formations; so that 
between each two consecutive formations, the forms of 
life have seldom changed in exactly the same degree. 
Yet if we compare any but the most closely related for¬ 
mations, all the species will be found to have undergone 
some change. When a species has onee disappeared 
from the face of the earth, we have reason to believe 
that the same identical form never reappears. Tho 
strongest apparent exception to this latter rule, is that 
of the so-called colonies ” of M. Barrande, which 
intrude for a period in the midst of an older formation, 
and then allow the pre-existing fauna to reappear; but 
LyelFs explanation, namely, that it is a case of tempo¬ 
rary migration from a distinct geographical province, 
seems to me satisfactory. 
p 
