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that in these ancient periods, near their points of origin, animals 

 found the earth comparatively unoccupied, and were not only able, 

 but in fact forced, to migrate in every direction into different habi- 

 tats, and to make perpetual efforts to readjust their inherited struc- 

 tures to the new requirements demanded by these comparatively 

 unoccupied fields. Food and opportunity would have acted, in 

 such localities, as stimulants to new efforts for the attainment of 

 more perfect adaptation and for changes of structure useful to that 

 end. We can neither imagine the effort to change of habitat and 

 consequently change of habits, without their cause the primary 

 physical stimulant of change in the environment, nor the changes 

 of structure, except as results^ of efforts on the part of the organ- 

 ism to meet the physical requirements of the surroundings. That 

 this process should end in the production of structures suited to the 

 environment is inevitable. With these factors at work, both without 

 and within the organism, the evolution of their structures obey a 

 physical law which acts amid a thousand disturbing forces perhaps, 

 but nevertheless must act with predominating force in one mean path 

 or direction, the resultant determined by the environment and the 

 inherited structures of the organism. 



One can compare the changes taking place during the whole of 

 Paleozoic time with those known to have occurred in certain iso- 

 lated cases in more recent times; such, for example, as that of 

 Steinheim, where a single species, finding itself in an unoccupied 

 field, proceeded with unexampled rapidity to fill it by the evolution 

 of new series and many species, all differing from each other, but 

 all referable, by intermediate varieties, to the original form — in this 

 example, a single species, the well-known Planorbis cequiumbilica- 

 tus.* 



The rapid evolution of the entire family of the Arietidae can also 

 be used to illustrate this point. This family originates from one 

 ancestral species and yet the process is so rapid that eleven distinct 

 series and seven genera arise, culminate and disappear within the 

 limits of a single age of geologic history, the Lower Lias of Europe, 

 South America and North America.*)* 



There are a number of other well-known cases, which could be 

 cited, illustrating the quick evolution of species ia locations which 



*" Genesis of Tertiary Species of Planorbis at Steinheim," by Alpheus Hyatt, Memoirs 

 50 Year Anniv. Bost. Soc. Nat. Hist. 



t "Genesis of the Arietidse," by A. Hyatt, Smithson. Contrib. No. 673, Mem. Mas. 

 Comp. Zool. 



