CHAP. xxi. * NATURAL SELECTION.' 413 



abstract of a much larger work not yet published, I could 

 not easily give an analysis of its contents within narrower 

 limits than those of the original, but it may be useful to 

 enumerate briefly some of the principal classes of phenomena 

 on which the theory of e Natural Selection ' is believed by 

 its author to throw light. 



In the first place, it would explain, says Mr. Darwin, the 

 unity of type which runs through the whole organic world, 

 and why there is sometimes a fundamental agreement in 

 structure in the same class of beings which is quite indepen 

 dent of their habits of life, for such structure, derived by 

 inheritance from a remote progenitor, has been modified, in 

 the course of ages, in different ways, according to the condi 

 tions of existence. It would also explain why all living and 

 extinct beings are united, by complex radiating and circuitous 

 lines of affinity with one another, into one grand system ; * 

 also, there having been a continued extinction of old races 

 and species in progress, and a formation of new ones by varia 

 tion, why in some genera which are largely represented, or to 

 which a great many species belong, many of these are closely 

 but unequally related; also, why there are distinct geographical 

 provinces of species of animals and plants, for, after long 

 isolation by physical barriers, each fauna and flora, by varying 

 continually, must become distinct from its ancestral type, 

 and from the new forms assumed by other descendants which 

 have diverged from the same stock. 



The theory of indefinite modification would also explain why 

 rudimentary organs are so useful in classification, being the 

 remnants preserved by inheritance of organs which the present 

 species once used as in the case of the rudiments of eyes 

 in insects and reptiles inhabiting dark caverns, or of the 

 wings of birds and beetles which have lost all power of flight. 



* Origin, p. 498. 



