IMPERFECTION OF PAL^EONTOLOGICAL RECORD. 2/ 



were opened between the Bohemian area and the general area 

 of Northern Europe, an immigration of species would take 

 place from the latter into the former area. The Upper 

 Silurian species of the latter area would thus be imported, in 

 greater or less numbers, into the midst of the general Lower 

 Silurian fauna of Bohemia, and would be preserved in the 

 Lower Silurian Rocks. If, however, the channel of com- 

 munication were speedily closed, so that the new-comers 

 could not be constantly reinforced by fresh immigrants, the 

 " colonial " species would die out, and the general Lower 

 Silurian fauna would again reign supreme. A reopening of 

 the channel of communication would allow of a fresh immigra- 

 tion and the formation of a fresh " colony," and the process 

 might be indefinitely repeated. Finally, however, we must 

 suppose that the Bohemian area was permanently thrown open 

 to immigration from the general European area, when the 

 Upper Silurian fauna of the latter would succeed in per- 

 manently and completely displacing the old Lower Silurian 

 fauna of the former region. The phenomenon, therefore, of 

 " colonies," may be defined as "the coexistence of two general 

 faunas, which, considered in their entirety, are nevertheless 

 distinct ; " and it is to be regarded as merely a case of migra- 

 tion under certain peculiar and exceptional circumstances. 



CHAPTER IV. 



THE IMPERFECTION OF THE PAL^ONTOLOGICAL 

 RECORD. 



As has been already pointed out, the series of the stratified 

 formations is an imperfect one, and is likely ever to remain so. 

 The causes of this " imperfection of the geological record," as it 

 has been termed by Darwin, are various ; but it is chiefly to be 

 ascribed to our as yet incomplete knowledge of the geology of 

 vast areas of the earth's surface, to denudation, and to the fact 

 that many of the missing groups are buried beneath other de- 

 posits, whilst more than half of the superficies of the globe is 

 hidden from us by the waters of the sea. The imperfection of 

 the geological record necessarily implies an equal imperfection 

 of the " palaeontological record;" but, in truth, the record of 

 life is far more imperfect than the mere physical series of de- 

 posits. As we are here chiefly concerned with the biological 



