'produced by great Geographical Changes. 391 



chiate Cephalopoda which supplied their food ; that the forms 

 of Reptilia preserved or developed during this period were those 

 (as the procoelian Crocodilia) which at the present day we find 

 subsisting under these new and different conditions; that in 

 several parts of the Southern Hemisphere there are still preserved 

 to us remnants of the warm-blooded fauna of the secondary period 

 in a state of isolation from different stages in that period ; and 

 that the disappearance of the wingless birds of the Trias, except 

 the Struthionidse, has taken place at the times when, and in the 

 places where, they were exposed to the attacks of the Felinse, or 

 other carnivora of power and activity equal to the mastery of 

 such powerful birds, or of man, and that the Struthionidse 

 alone, by their superior means of escape, have withstood these 

 enemies. 



The topics discussed in this paper are in harmony with Mr. 

 Darwin's law of natural selection. The unequal rate at which 

 some forms of mammalia have, when compared with others, 

 changed their generic and specific characters, and even those of 

 the suborders to which they belonged, as shown in the compa- 

 rison of the change in the Opossum and Macacus with the true 

 Ungulata, appears to be due to the greater influence which 

 changing external conditions have upon some than upon other 

 forms of life. The competition for existence, and the consequent 

 elimination of new types of being, has had its maximum upon 

 the Europeo-Asiatic continent -, but the process has had its more 

 limited parallel in the circumscribed regions formed by the iso- 

 lated remnants of the secondary continents ; for while the Lo- 

 phiodontia, Solipedia, Ruminantia, Carnivora, and Pachydermata 

 were eliminated in the former, a corresponding development of 

 being in Australia, limited to the one order Marsupialia, took 

 place in the introduction of the Macropi, Nototheria, Wombats, 

 and other forms of Australian life, existing and extinct, analogous 

 in their habits and powers with the characteristic mammalia of 

 the Europeo-Asiatic continent, accompanied by a formidable car- 

 nivorous type, and in South America by the order Bruta and 

 its allies. 



The occurrence in Australia and its contiguous islands, and in 

 Madagascar, of existing forms having the nearest affinities to the 

 secondary Mammalia, and of the sole survivors of the once abun- 

 dant Cestraciont fishes and mollusk Trigonia, appears to me 

 susceptible of more rational explanation on the ground that in 

 these regions we have preserved to us isolated tracts which once 

 formed parts of the continents of the secondary period, in which 

 a portion of the secondary inhabitants have, as it were, been im- 

 prisoned, than on the ground that the fauna of the secondary 

 periods, and that of Australia, was and is respectively that most 



