io6 DISCRIMINATIONS CHAP. 



tinctive characters, down to the minutest 

 ornamentation on a scale or on an 

 osseous scute, or to the peculiar varieties 

 of pattern on the convolutions of an 

 Ammonite. These species continue till 

 they die, and then they are often suddenly 

 replaced by new forms and new patterns, 

 all as definite and as persistent as before. 

 How this takes place no man as yet can 

 tell. 



I recollect one striking illustration. 

 Some thirty-five years ago I visited the 

 distinguished French geologist Barrande, 

 who devoted himself for years to the life- 

 history of the Trilobites in the Silurian 

 rocks of Bohemia. He had a magnifi- 

 cent collection of those curious crusta- 

 ceans in his house in Prague. Nothing 

 was more remarkable than the stability 

 of the forms which he identified. This 

 stability extended to the immature or 

 larval forms of each species. He had 



