THE RECAPITULATION THEORY 297 



and organs. It derives also strong support from 

 the ready explanation which it gives of many 

 otherwise unintelligible points. 



Of these latter a familiar and most instructive 

 instance is afforded by rudimentary organs i.e., 

 structures which, like the outer digits of the horse's 

 leg, or the intrinsic muscles of the ear of a man, 

 are present in the adult in an incompletely devel- 

 oped form, and in a condition in which they can be 

 of no use to their possessors or else structures 

 which are present in the embryo, but disappear 

 completely before the adult condition is attained, 

 for example the teeth of whalebone whales, or the 

 branchial clefts of all higher vertebrates. 



Natural Selection explains the preservation of 

 useful variations, but will not account for the 

 formation and perpetuation of useless organs ; and 

 rudiments such as those mentioned above would be 

 unintelligible but for Recapitulation, which solves 

 the proBIem at once, showing that these organs, 

 though^ now useless, must have been of functional 

 valire^Tolhe ancestors of their present possessors, 

 and that their appearance in the ontogeny of existing 

 forms is due to repetition of ancestral characters. 

 Such rudimentary organs are, as Darwin pointed 

 out, of larger relative or even absolute size in the 

 embryo than in the adult, Because the embryo 

 represents the stage in the pedigree in whiclTthey 

 were functionally activer^ 



Rudimentary^ergans are extremely common, 

 especially among the higher groups of animals, and 

 their presence and significance are now well under- 



