-2 THE STORY OF THE EARTH. 



recognise the technical characters by which ani- 

 ld plants are classified, arc aware that 

 re different kinds i op shells, and 



different kinds of cockles. The change in form 

 ornament can often be seen to originate as a 

 sequence of the heme of the shells being a 

 e where the water is still, or one where it is 

 eptionally disturbed, the ribs of shells being 

 ays stronger in rough water. The presence 

 of fresh water in an estuary would appear to be 

 a frequent caiise of variation, not only in orna- 

 ment, but in form. Such variations of the com- 

 mon periwinkle and purple shell are found in 

 g-beds at Norwich, and seen in inlets on the 

 5t at the present day. Many of these varia- 

 tion- - ich as might characterize different 

 genera if they were persistent and became per- 

 manent characters, but they do not even consti- 

 tute s, and are only regarded as local races 

 due to local causes. If it were possible that after 

 • had come into existence, another set 



of circumstances affected it so as to cause varia- 

 tion, to tak< m some new direction, it may 

 that what was previously but a race charai 

 ild be perpetuated in all the new modifica- 

 tions, and become the distinctive attribute of a 

 even of a genus. The capacity of an 



animal for variation is usually in the dcvclop- 



omething new, which did not previ- 



; but the most remarkable evidences 



e in the loss of parts which had 



in animals in a previous period of time. 



variation is strikingly seen in 



become 



ment of 



state of know ledge 



