CHAPTER VII. 



GEOGRAPHICAL DISTRIBUTION : SPECIAL CONSIDERA- 

 TION: THE ADJUSTMENT OF ORGANISMS TO EN- 

 VIRONMENT. 



R6sum6. In the case of the Madreporarian corals it was 

 observed that as geological time progressed new genera actu- 

 ally were initiated, and the succession of genera and the rate 

 of their increase was seen to be definitely associated with suc- 

 cession of time. Likeness of structure and likeness of time, 

 dissimilarity of form and separation in time, slowness or 

 rapidity of initiation of new genera, and a particular geologi- 

 cal period of time for each family, order, or class, are inter- 

 preted to mean that there is a definite relationship existing 

 between differentiation of structure and passage of time. 

 This we assume to be a law of the order of events, and we 

 infer the general hypothesis that the form and structure of 

 organisms of one geological period are in some measure deter- 

 mined by the form and structure of the organisms of the 

 period immediately preceding. 



This hypothesis involves two particular propositions : 



(1) That each organism is genetically related to some pre- 

 existing ancestor whose form and structure were not exactly 

 like its own. 



(2) That the process of organic reproduction is not a stereo- 

 type process of repeating in the offspring the exact characters of 

 the ancestry, but that the production of differences between the 

 parent and offspring is a normal factor in the reproductive 

 process, either continuously or occasionally in operation. 



There is, however, another fact to be noted : the innu- 

 merable differences in the conditions of environment are more 

 or less distinctly expressed by differences in the kinds of 

 organisms associated with them. All kinds of animals are not 



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