68 OBGANIC EVOLUTION CONSIDERED 



two tendencies above named, the tendency of the off- 

 spring to closely resemble the parent, and the tend- 

 ency to vary somewhat from the parent. The 

 essence of the theory of orgauic evolution is involved 

 in the relative strength of these tendencies. They 

 may be compared to the centripetal and thecentrif- 

 agul forces. 



Is there an orbit of variation for each species be- 

 yond which the tendency to variation cannot carry the 

 form? Is the tendency to resemble the parent a 

 centripetal force that can forever hold the amount of 

 variation within a definite orbit? or, on the other 

 hand, does the centrifugal force carry the new forms 

 off in tangents so that there is no return to the 

 ancestral form? 



It is evident that if the theory of natural selection 

 is true, it is not a complete theory of organic evolu- 

 tion. It accounts only for preserving certain forms 

 instead of others. Darwin says, " Natural selection 

 acts solely through the preservation of variations in 

 some way advantageous, which consequently endure." 



Again, he says, " I have called this principle by 

 which each slight variation, if useful, is preserved, by 

 the term Natural Selection, in order to mark its rela- 

 tion to man's power of selection." 



Natural Selection attempts, therefore, to account 

 for the preservation of certain variations, and not foi 

 their production. As to the causes and the amount 

 of variation, no satisfactory theory has been given. 

 Darwin, after assigning various reasons why he thinks 

 variations may take place, says, " Our ignorance ol 

 the laws of variation is profound. Not in one case 

 out of a hundred can we pretend to assign any reasoc 

 why this or that part has varied." And so it happens 

 that we are left in almost total darkness as to the 

 cause of the most important factor in organic evolu- 



