46 PROFESSOR A. M. MARSHALL. 



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 

 notochord, 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 gradually replaced by 

 other and unlike structures by substitution. The notochord is an 

 intermediate organ, and the cartilaginous skeleton which replaces 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 is a 

 better weapon than a stick or stone ; it is used for the same purpose, 

 and the importance or need for a better weapon led to the replacement 

 of the sling by the bow ; the bow does not arise by further develop- 

 ment or 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 these 

 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 evolution of an organ is necessary as a stimulus to the develop- 

 ment of the next succeeding stage, then it becomes clear why animals 

 are constrained to recapitulate. 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 in- 

 dispensable as a stimulus to the development of the permanent 

 structures of the adult. 



