CHAPTER VI. 



GEOGRAPHICAL DISTRIBUTION THE GENERAL RELA- 

 TION OF ORGANISMS TO THE CONDITIONS OF ENVI- 

 RONMENT. 



IN the last chapter it was shown by an analysis of the 

 characters of the genera of Madreporaria a group of organ- 

 isms well adapted to furnish this evidence (because of their 

 living under the same conditions required for the making of 

 the strata themselves, and producing hard parts, easily pre- 

 served from the earliest times onward) that the form of an 

 organism has an intimate relationship to the geological period 

 during which it lived. 



The natural conclusion from this observation is that the 

 order of sequence in the appearance of organisms is the ex- 

 pression of a natural law of their succession in time, or that 

 it is a law of nature for organisms to succeed each other in 

 this observed geological order. 



We observed that the classification of organisms by their 

 morphological characters, as expressed in their arrangement 

 in the classes, orders, families, and genera of the zoologist, 

 shows that this relation of characters to time of appearance is 

 expressed in every detail of structure, and the more minute 

 our inspection the more distinctly is the truth of this princi- 

 ple brought to light. 



A species or genus has not only a particular relationship 

 to other species or genera, but every genus has a particular 

 period in the time-scale when it lived, and a particular dura- 

 tion of geological time to which its living was limited, before 

 which it did not exist, and after which it failed to reappear. 

 This illustrates the general law that the particular morphologi- 

 cal characters assumed by an individual organism are immedi- 

 ately related to the ancestry which is behind it; but if we turn 

 our attention to the facts of geographical distribution, we 



in 



