THE STUDY OF STRUCTURE. 341 



about organs as if tliej 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 ascidians and lancelets 

 is almost wholly subservient to respiration. More- 

 over, in organs which are not very highly specialised, 

 it seems as if the component elements retained a con- 

 siderable degree of individuality, so that in course of 

 time what was a secondary function may become the 

 primary one. Thus Dohrn, who has especially em- 

 phasised the 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 diminu- 

 tion 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 th^ organ." We 

 may 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 noto- 



