38 THE ELEMENTS OF STRUCTURE. 



Change of function. Division of labour involves restric- 

 tion of functions in the several parts of an animal, and no 

 higher Metazoa could have arisen if all the cells had 

 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 has 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." Thus it may be noticed that the 

 structure known as the allantois is an unimportant bladder 

 in the frog, that in Birds and Reptiles it forms a foetal 

 membrane (chiefly respiratory) around the embryo, and that 

 in most Mammals it forms part of the placenta which effects 

 vital connection between offspring 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 

 wood. Yet it remains as the sole axial skeleton in 



