THE RECAPITULATION THEORY 359 



by substitution. He points out that any modi- 

 fication of an organ or tissue must involve 

 modification, at least in functional activity, of other 

 organs. He then continues by urging that one 

 organ may replace or be substituted for another, 

 the replacing organ being in no way derived 

 morphologically from the replaced or preceding 

 organ, but having a genetic relation to it of this 

 kind : that it can only arise in an organism so 

 constituted, and is dependent on the prior existence 

 of the replaced organ, which supplies the necessary 

 stimulus for its formation. As an example he 

 takes the axial skeleton of vertebrates. The noto- 

 chord, formed by change of function from the wall 

 of the digestive canal, is the sole skeleton of the 

 lowest vertebrates, and the earliest developmental 

 phase in all the higher forms. The notochord 

 gives rise directly to no other organ, but is gradu- 

 ally replaced by other and unlike structures by 

 substitution. The notochord is an intermediate 

 organ, and the cartilaginous skeleton which re- 

 places it is only intelligible through the previous 

 existence of the notochord ; while, in its turn, the 

 cartilaginous skeleton gives way, being replaced, 

 through substitution, by the bony skeleton. 



The successive phases in the evolution of weapons 

 might be quoted as an illustration of Kleinenberg's 

 theory. The__bow and arrow are a better weapon 

 than a stick or stone ; they are used for the same 

 purpose, and the importance or need for a better 

 weapon led tojhe replacement of the sling by the 

 bow. The bow does not arise by further develop- 



