34 THE ELEMENTS OF STRUCTURE. 



Change of Function. 



Division of labour involves restriction 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, and we know how wondrously diverse are the 

 activities in our brains. 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 considerable 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." 

 Notice, in illustration, how the structure known as the 

 allantois is an unimportant bladder in the frog, while in 

 Birds and Reptiles it forms a foetal membrane (chiefly 

 respiratory) around the embryo, and in most Mammals 

 forms part of the placenta which effects nutritive 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 Kleinenberg. 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 backbone, but is replaced 



