£90 GROUPS OF ALLIED SPECIES Cuap. IX. 



as they are at present. Even at tliis day, if the Malay Archi 

 pehigo "svcre converted into land, the tropical parts of the In- 

 dian Ocean would form a large and perfectly-enclosed basin, 

 in Avliich any great group of marine animals might be multi- 

 plied ; and 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 considerations, from our ignorance of the geol- 

 ogy of other countries beyond the confines of Europe and the 

 United States, and fi'om the revolution in our paleontological 

 knowledge eflected by the discoveries of the last dozen years, 

 it seems to me to be about as rash to dogmatize on the succes- 

 sion of organic forms throughout the world, as it would be for 

 a naturalist to land for five minutes on a barren point in Aus- 

 tralia, and then to discuss the number and range of its jjroduc- 

 tions. 



On the sudden A2)2)earance of Groups of allied Species in 

 the loicest Jcnoicn Fossil if eroiis Strata. 



There is another and allied difficulty, which is much more 

 serious. I allude to the manner in Avhich many species in sev- 

 eral of the main divisions of the animal kingdom 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 are descended from a single pro- 

 genitor, apply with nearly equal force to the earliest known 

 species. For instance, it cannot be doubted that all the Silu- 

 rian trilobites are descended from some one crustacean, which 

 must have lived long before the Silurian age, and which prob- 

 ably difl'ered greatly from an>' known animal. Some of the 

 most ancient Silurian animals, as the Nautilus, Lingula, etc., 

 do not difter much from living sjiccies; and it cannot on our 

 theory be sujiposed that these old species were the progenitors 

 of all the species belonging to the same groups whicli have 

 subseciucntly appeared, for they are not in any degree inter- 

 mediate in character. 



Consequently, if the tlicory be true, it is indis]nitable 

 that, before the lowest Silurian or Cambrian stratum was do- 

 posited long periods elapsed, as long as, or probably far longer 

 than, tlie whole interval from the Cambrian age to the present 

 day; I'.nd that during these vast periods the world swarmed 



