36 THE ELEMENTS OF STRUCTURE. 



remained with the many-sided qualities of Amcebse. 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." 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 fcetal 

 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 

 Amphioxus, persists in great part in hag and lamprey, but 

 becomes less and less persistent in Fishes and higher 



