1 88 The Evolution Hypothesis. 



and of modifications that, by hypothesis, have been 

 growing ever more complex. It is impossible to con- 

 struct a rational theory on other terms. On these 

 conditions only can a philosophy of evolution be esta- 

 blished. The evolutionist must not only prove that 

 all organisms have been gradually differentiated 

 through continuous successive changes ; he must also 

 account for the process. 



The facts open to observation lie within a brief 

 period of the history of the universe. No more than 

 a narrow strip across the pattern which is being 

 woven in the loom of time is visible to man. From this 

 restricted experience he is left to trace the design back- 

 ward to the far distant beginning. The instances out 

 of which all theories as to the origin of organic forms 

 are framed may be divided into two classes, organisms 

 as found at the present time with their resemblances 

 and differences, and distribution ; and the facts of 

 organic history in so far as they are revealed in the 

 geological record. Over the entire field there has not 

 been discovered anywhere direct evidence of transition 

 form a less developed to a more highly developed 

 species. That the earlier are the lower in organization 

 is to be looked for on any hypothesis. If we accept 

 the geologist's account of the history of the earth's 

 crust, a gradual advancement from lower to higher 

 forms is inevitable. No species can exist except in a 

 suitable habitat ; and the earlier geological periods did 

 not afford terrestrial conditions adapted to beings 



