406 THE INDUCTIONS OF BIOLOGY. 



strata gives proof of continued existence of the type in the 

 locality ; while they are comparatively large and abrupt where 

 the adjacent formations are shown to have been separated 

 by long intervals. 



Another general fact, referred to by Mr. Darwin as one 

 which palaeontology has made tolerably certain, is that forms 

 and groups of forms which have once disappeared from the 

 Earth, do not reappear. Passing over the few species which 

 have continued throughout the whole period geologically 

 recorded, it may be said that each species after arising, spread- 

 ing for an era, and continuing abundant for an era, eventually 

 declines and becomes extinct; and that similarly, each genus 

 during a longer period increases in the number of its species, 

 and during a longer period dwindles and at last dies out. 

 After making its exit neither species nor genus ever re-enters. 

 The like is true even of those larger groups called orders. 

 Four types of reptiles which were once abundant have not 

 been found i'n modern formations, and do not at present exist. 

 Though nothing less than an exhaustive examination of all 

 strata, can prove conclusively that a type of organization 

 when once lost is never reproduced, yet so many facts point 

 to this inference that its truth can scarcely be doubted. 



To frame a conception of the total amount and general 

 direction of the change in organic forms during the time 

 measured by our sedimentary series, is at present impossible — 

 the data are insufficient. The immense contrast between the 

 few and low forms of the earliest-known Fauna, and the 

 many and high forms of our existing Fauna, has been com- 

 monly supposed to prove, not only great change but great 

 progress. Nevertheless, this appearance of progress may be, 

 and probably is, mainly illusive. Wider knowledge has shown 

 that remains of comparatively well-organized creatures really 

 existed in strata long supposed to be devoid of them, and 

 that where they are absent, the nature of the strata often 

 explains their absence, without assuming that they did not 

 exist when these strata were formed. It is a tenable 



