PREFACE ix 



coveries extending over a long series of years, and is 

 perhaps the first attempt yet made to trace in detail the 

 origin, growth, and final decay of that puzzling object, 

 the common flint. 



The " Origin of Freshwater Animals," * which in its 

 present form is also new, is based on a communication 

 made to the Eoyal Dublin Society some years ago. Its 

 object is to explain how marine animals contrived to 

 obtain a home in rivers and lakes, or how they became 

 the captives of the freshwater domain : the shifts they 

 are put to in maintaining their foothold, and the com- 

 plications which result. A passing reference is made to 

 the fauna of lake Tanganyika. 



The "Influence of Oxford on Geology " was delivered 

 as an Inaugural lecture on assuming the duties of the 

 chair of Geology in the University ; it has already appeared 

 in Science Progress. The subject chiefly treated is the 

 early history of geology, when it was engaged in a life- 

 and-death struggle over the nature of fossils. Opportunity 

 is taken to do justice to the genius of that truly great 

 observer, Steno of Bologna. 



The " Key to Terrestrial History," which appears here 

 for the first time, was delivered as a lecture before the 

 Koyal Institution. It gives some account of the great 

 progress made during the last few decades towards 

 establishing the universality of geological epochs and 

 episodes, and endeavours to countervail the damaging 

 effects produced by the famous criticisms of Huxley and 

 Herbert Spencer while at the same time vindicating the 

 validity of the fundamental discovery of William Smith. 



"Geologies and Deluges" was a British Association 

 lecture to working men, delivered at the meeting in 

 Oxford, and subsequently published in Nature. After 



* " On the Origin of Freshwater Faunas : a Study in Evolution," 

 Sic. Trans. Roy. Dub. Soc., ser. II., vol. Hi. p. 87, 1884. 



