250 NATURAL HISTORY. 



extinction; but he thinks that they have been exter- 

 minated by man. 



On the other hand, in the case of marine animals, such 

 as shell-fish, where human agency certainly cannot have 

 caused extinction, he has a different theory. Such species, 

 he says, should not be regarded as really extinct, but 

 should rather be considered as belonging to existing 

 species in the sense, namely, that the living types are 

 only the modified descendants of the fossil forms. The 

 real fact which, in his opinion, ought to astonish us is to 

 find among fossils any species which are identical with 

 living forms. 'This fact, which our collections place 

 beyond doubt, ought to lead us to the belief that the 

 fossil remains of animals which are still represented in the 

 living state, are those of which the antiquity is least. The 

 species to which such forms belong have, doubtless, not 

 yet had time to permit of their having varied to any great 

 extent.' Lamarck, therefore, had got hold of the prin- 

 ciple which Lyell afterwards employed with such effect in 

 his well-known classification of the Tertiary rocks. 



Not only did Lamarck hold that the existing species of 

 animals had been produced by the gradual modification of 

 pre-existing species ; but he thought that the course of 

 modification had been, on the whole, a progressive and 

 not a retrograde one. He believed that the simpler forms 

 of life had been produced first, and the more complex 

 ones later Man being the last evolved and the highest of 

 all animals. In order to account, however, for the exist- 

 ence at the present day of any simple or degraded types 

 of animal life, Lamarck thought it necessary to assume 

 that such simple forms were always being produced afresh 



