306 IMPERFECTION OF THE Chap. IX. 



here they would remain confined, until some of the 

 species became adapted to a cooler climate, and were 

 enabled to double the southern capes of Africa or 

 Australia, and thus reach other and distant seas. 



From these and similar considerations, but chiefly 

 from our ignorance of the geology of other countries 

 beyond the confines of Europe and the United States ; 

 and from the revolution in our palseontological ideas 

 on many points, which the discoveries of even the last 

 dozen years have effected, it seems to me to be about 

 as rash in us to dogmatize on the succession of organic 

 beings throughout the world, as it would be for a 

 naturalist 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 knoivn 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 fossiliferous 

 rocks. Most of the arguments which have convinced 

 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 probably 

 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. 



