PART II. 



GEOLOGICAL DISTRIBUTION. 



I. 



The succession of life. — Faunas of the different geological periods. 



There is no fact more patent in the history of the organic 

 world than that there has been from first to last a progressive evo- 

 lution from lower to higher forms in the chain of beings that suc- 

 cessively peopled the earth's surface. Casting our eye back over 

 the vast series of rock deposits which together constitute the fossili- 

 ferous scale of geologists, from the Cambrian to the Post-Pliocene, 

 and which together have a maximum development of probably 

 not less than two hundred thousand feet (or forty miles), we re- 

 mark along the most ancient horizon the traces of animals which 

 bespeak the organisation of some of the lowest forms of life with 

 which we are at present acquainted; in the middle distance we 

 note the appearance of forms whose organisation marks a decided 

 advance upon that of their predecessors ; and, finally, in the fore- 

 ground, we are brought upon the threshold of those highly com- 

 plicated forms which to-day people the surface of the earth. The 

 simpler forms of life came into existence first; the most complex 

 last. It must not be implied, however, that with the progressive 

 and steady evolution of higher forms there has been an equally 

 progressive destruction or elimination of the forms of lower or- 

 ganisation; both have kept pace with each other, so that, at the 

 present day, although innumerable groups have completely disap- 

 peared, the lowest is found flourishing side by side with the highest. 

 This inter-association of lower and higher forms has manifested it- 



