360 THE RECAPITULATION THEORY 



mentor increasing perfection of the sling ; it is 

 an entirely new weapon, towards the formation 

 of which the older and more primitive weapons 

 have acted as a stimulus, and which has replaced 

 tftese latter by substitution, while the substitution 

 at a later date of firearms for the bow and arrow 

 is merely a further instance of the same principle. 



It is too early yet to realise the full significance 

 of Kleinenberg's most suggestive theory ; but if it 

 be really true that each historic stage in the evolu- 

 tion of an organ is necessary as a stimulus to the 

 development of the next succeeding stage, then it 

 becomes clear why animals are constrained to re- 

 capitulate. Kleinenberg suggests further that the 

 extraordinary persistence in embryonic life of 

 organs which are rudimentary and functionless in 

 the adult may also be explained by his theory, the 

 presence of such organs in the embryo being indis- 

 pensable as a stimulus to the development of the 

 permanent structures of the adult. It would be 

 easy to point out difficulties in the way of the 

 theory. (_JThe omission of historic stages in the 

 actual ontogenetic development, of which almost 

 all groups of animals supply striking examples, is 

 one of the most serious ; for if these stages are 

 necessary as stimuli for the succeeding stages, 

 then^ their omission requires explanation ; while if 

 such stimuli are not necessary, the theory would 

 appear to need revision. Such objections may 

 however prove to be less serious than they appear 

 at first sight ; and in any case Kleinenberg's 

 theory may be welcomed as an important and 



