436 THE EVOLUTION OF LIFE. 



able in direct proof that by progressive modifications, races of 

 organisms which are apparently distinct from antecedent 

 races have descended from them, are not sufficient ; yet there 

 are numerous facts of the order required. Beyond all ques- 

 tion unlikenesses of structure gradually arise among the 

 members of successive generations. We find that there is 

 going on a modifying process of the kind alleged as the 

 source of specific differences : a process which, though slow, 

 does, in time, produce conspicuous changes — a process which, 

 to all appearance, would produce in millions of years, any 

 amount of change. 



In the chapters on ^'Heredity" and "Variation," contained 

 in the preceding Part, many such facts were given, and more 

 might be added. Although little attention has been paid to 

 the matter until recent times, the evidence already collected 

 shows that there take place in successive generations, altera- 

 tions of structure quite as marked as those which, in succes- 

 sive short intervals, arise in a developing embryo — nay, often 

 much more marked; since, besides differences due to changes 

 in the relative sizes or parts, there sometimes arise differ- 

 ences due to additions and suppressions of parts. The struc- 

 tural modification proved to have taken place since organisms 

 have been observed, is not less than the hypothesis demands 

 — bears as great a ratio to this brief period, as the total 

 amount of structural change seen in the evolution of a com- 

 plex organism out of a simple germ, bears to that vast period 

 during which living forms have existed on the Earth. 



We have, indeed, much the same kind and quantity of 

 direct evidence that all organic beings have arisen through 

 the actions of natural causes, which we have that all the 

 structural complexities of the Earth's crust have arisen 

 through the actions of natural causes. Between the known 

 modifications undergone by organisms, and the totality of 

 modifications displayed in their structures, there is no greater 

 disproportion than between the observed geological changes, 

 and the totality of geological changes supposed to have been 



