88 THE GEOLOGICAL EVIDENCES 



far been discovered. But beyond this it would be 

 dangerous to venture. 



We have thus far confined our attention 

 exclusively to a consideration of the higher 

 groups of animals, the Vertebrata. The lower 

 or invertebrate animals present equally striking 

 proofs of modification and transformism, but the 

 limited time at my command will permit me 

 to bring before you only one or two special 

 cases, drawn from the class of Mollusca, with 

 which my own investigations are connected. If 

 the doctrine of evolution holds true, it stands 

 to reason that, as in the case of higher animals, 

 the existing fauna must be foreshadowed in 

 the fauna of a period immediately preceding ; 

 this connection cannot generally be established, 

 owing probably to migrations and intermixtures 

 of different faunas, as depending upon changes 

 in the physical condition of the surroundings. 

 In the sheltered region' of the Gulf of Mexico, 



