418 Notices of Memoirs — British Association — 



Cretaceous period, and the Mosasauria especially must have been, 

 extremely abundant and flourishing. Nevertheless, at the end of 

 Cretaceous times they disappeared everywhere, and there was 

 absolutely nothing to take their place until the latter part of the 

 Eocene period, when whales and porpoises began to play exactly 

 the same part. So far as we know, the higher race never even came 

 in contact with the lower race ; the marine mammals found the seas 

 vacant, except for a few turtles and for one curious E,hynchocephalian 

 reptile (Champsosmirus), which did not long survive. Another 

 illustration of the same phenomenon is probably afforded by the 

 primitive Carnivora (the so-called Sparassodonta), which were 

 numerous in South America in the Lower Tertiary periods. Thej^ 

 were animals with a brain as small as that of the thylacines and 

 dasyures which now live in Tasmania. They appear to have died 

 out completely before they were replaced by the cats, sabre-toothed 

 tigers, and dogs, which came down south from IS'orth America over 

 the newly emerged Isthmus of Panama at the close of the Pliocene 

 period. At least, the remains of these old carnivores and their 

 immigrant successors have never yet been found associated in any 

 geological formation. 



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I am therefore still inclined to believe that the comparison of vital 

 processes with certain purely physical phenomena is not altogether 

 fanciful. Changes towards advancement and fixity which are so 

 determinate in direction, and changes towards extinction which are so 

 continually repeated, seem to denote some inherent property in living 

 things, which is as definite as that of crystallization in inorganic 

 substances. The regular course of these changes is merely hindered 

 and modified by a succession of checks from their environment and 

 by I^atural Selection. Each separate chain of life, indeed, bears a 

 striking resemblance to a crystal of some inorganic substance which 

 has been disturbed by impurities during its growth, and has thus been 

 fashioned with unequal faces, or even turned partly into a mere con- 

 cretion. In the case of a crystal the inherent forces act solely on 

 molecules of the crystalline subject itself, collecting them and striving, 

 even in a disturbing environment, to arrange them in a fixed 

 geometrical shape. In the case of a chain of life (or organic phylum) 

 we may regard each successive animal as a temporary excrescence 

 of colloid substance round the equally colloid germ-plasm which 

 persists continuously from generation to generation. The inherent 

 forces of this germ-plasm, therefore, act upon a consecutive series 

 of excrescences (or animal bodies), struggling, not for geometrically 

 arranged boundaries, but towards various other symmetries, and 

 a fixity in number of multiple parts. When the extreme has been 

 reached, activities cease, and sooner or later the race is dead. 



Such are some of the most important general results to which the 

 study of fossils has led during recent years ; and they are conclusions 

 which every new discovery appears to make more certain. 



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Serious difficulties have also become apparent during recent years in 



