Chap. IX. 
GEOLOGICAL RECORD. 
307 
beings throughout the world, as it would be for a natu¬ 
ralist to land for five minutes on some one barren point 
in Australia, and then to discuss the number and range 
of its productions. 
On the sudden appearance of groups of Allied Species 
in the lowest known fossiliferous strata. —There is another 
and allied difficulty, which is much graver. I allude to 
the manner in which numbers of species of the same 
group, suddenly appear in the lowest known fossili¬ 
ferous rocks. Most of the arguments which have con¬ 
vinced me that all the existing species of the same 
group have descended from one progenitor, apply with 
nearly equal force to the earliest known species. For 
instance, I cannot doubt that all the Silurian trilobites 
have descended from some one crustacean, which must 
have lived long before the Silurian age, and which pro¬ 
bably differed greatly from any known animal. Some 
of the most ancient Silurian animals, as the Nautilus, 
Lingula, &c., do not differ much from living species; 
and it cannot on my theory be supposed, that these old 
species were the progenitors of all the species of the 
orders to which they belong, for they do not present 
characters in any degree intermediate between them. 
If, moreover, they had been the progenitors of these 
orders, they would almost certainly have been long 
ago supplanted and exterminated by their numerous 
and improved descendants. 
Consequently, if my theory be true, it is indisputable 
that before the lowest Silurian stratum was deposited, 
long periods elapsed, as long as, or probably far longer 
than, the whole interval from the Silurian age to the 
present day; and that during these vast, yet quite un¬ 
known, periods of time, the world swarmed with living 
creatures. 
