38 THE ELEMENTS OF STRUCTURE. 



remained with the many-sided qualities of Amoebae. Yet 

 we must avoid thinking about organs as if they were 

 necessarily active in one way only. For many organs, e.g. 

 the liver, have several very distinct functions. In addition 

 to the main function of an organ, there are often secondary 

 functions ; thus the wings of an insect may be respiratory 

 as well as locomotor, and part of the food canal of Tunicates 

 and Amphioxus is almost wholly subservient to respiration. 

 Moreover, in organs which are not very highly specialised, 

 it seems as if the component elements retained a consider- 

 able degree of individuality, so that in course of time what 

 was a secondary function may become the primary one. 

 Thus Dohrn, who especially emphasised this idea of 

 function change, says : " Every function is the resultant of 

 several components, of which one is the chief or primary 

 function, while the others are subsidiary or secondary. 

 The diminution of the chief function and the accession of a 

 secondary function changes the total function ; the secondary 

 function becomes gradually the chief one ; the result is the 

 modification of the organ." The contraction of a muscle is 

 always accompanied by electric changes, and in the electric 

 organs of fishes we see the electric changes m the modified 

 muscular tissue composing the organs becoming more 

 important than the contractility. The structure known as 

 the allantois is an unimportant bladder in the frog, in Birds 

 and Reptiles it forms a foetal membrane (chiefly respiratory) 

 around the embryo, and in most Mammals it forms part of 

 the placenta which effects vital connection between off- 

 spring and mother. 



Substitution of organs. The idea of several changes of 

 function in the evolution of an organ, suggests another of 

 not less importance which has been emphasised by Kleinen- 

 berg. An illustration will explain it. In the early stages 

 of all vertebrate embryos, the supporting axial skeleton is 

 the notochord, a rod developed along the dorsal wall of 

 the gut. From Fishes onwards, this embryonic axis is 

 gradually replaced in development by the vertebral column 

 or backbone; the notochord does not become the back- 

 bone, but is replaced by it. It is a temporary structure, 

 around which the vertebral column is constructed, as a tall 

 chimney may be built around an internal scaffolding of 



