2io Instinct and Intelligence 



may be sure that our chalk rocks, for instance, 

 consist of the shells of marine species of 

 animals, and that these remains of once living 

 beings must have taken long periods of time to 

 have been deposited layer upon layer at the 

 bottom of the sea. Darwin states that the fine- 

 ness of gradation in the shells of successive sub- 

 stages of the chalk formations led him to main- 

 tain the gradual, as against the sudden evolu- 

 tion of species. The fossil shells of these rocks 

 have been thoroughly investigated by Mr. 

 A. W. Rowe, who observes that "the white 

 chalk of England offers an almost unique field 

 for observation on account of its thickness (con- 

 siderably over 1,000 feet), its slow, uniform, and 

 continuous deposit in a sea of moderate depth, 

 with no closely adjacent land, the abundance 

 and wonderful state of preservation of its 

 fossils, together with the facility with which 

 they can be cleared of their chalky covering." 

 Among the most common chalk fossils is the 



o 



flattened heart-shaped Sea-Urchin. These are 

 first found in their shelled sparsely ornamented 

 forms, from which spring as we ascend the 

 zone all the other species of the genus. The 



