PRINCIPLE OF FUNCTION-CHANGE 277 



function as walking limbs. In the evolution of Vertebrates 



o 



from Annelids the principle came into action in many con- 

 nections in the formation of a new mouth from gill-slits, in 

 the transformation of gills into fins and limbs, of segmental 

 organs into gill-slits, and so on. Dohrn tells us that the 

 principle of function - change was suggested to him by 

 Mivart's Genesis of Species (1870), and he points out how 

 it enables a partial reply to be made to the dangerous 

 objection raised against the theory of natural selection that 

 the first beginnings of new organs are necessarily useless in 

 the struggle for existence. 



We may note in passing that a somewhat similar idea 

 was later applied by Kleinenberg to the explanation of some 

 of the ancestral features of development. He pointed out in 

 his classical memoir on the embryology of the Annelid 

 Lopadorhynchus^ that many embryonic organs seem to be 

 formed for the sole purpose of providing the necessary 

 stimulus for the development of the definitive organs. Thus 

 the notochord is the necessary forerunner of the vertebral 

 column, cartilage the precursor of bone. " From this point 

 of view," he writes, " many rudimentary organs appear in 

 a different light. Their obstinate reappearance throughout 

 long phylogenetic series would be hard to understand 

 were they really no more than reminiscences of bygone 

 and forgotten stages. Their significance in the processes 

 of individual development may in truth be far greater 

 than is generally recognised. When in the course of the 

 phylogeny they have played their part as intermediary 

 organs ( Vermittelungsorgane] they assume the same function 

 in the ontogeny. Through the stimulus or by the aid of 

 these organs, now become rudimentary, the permanent parts 

 of the embryo appear and are guided in their development ; 

 when these have attained a certain degree of independence, 

 the intermediary organ, having played its part, may be 

 placed upon the retired list." 



Dohrn was well aware of the functional, or as he calls 



1 Zcits.f.iviss. Zool., xliv., 1886. 



Quoted by E. B. Wilson, Wood's Holl Biological Lectures for 1894, 



p. 121. 



