44 THE ELEMENTS OF STRUCTURE 



they are analogous but not homologous. Yet two organs 

 may be both homologous and analogous^ e.g. the wing of a 

 bird and the wing of a bat, for both are fore-Umbs, and 

 both are organs of flight. Sometimes two organs or two 

 organisms — deeply different in structure — have a marked 

 superficial resemblance, simply because both have arisen 

 in relation to similar conditions of life. Thus a burrow- 

 ing amphibian, a burrowing lizard, and a burrowing snake 

 resemble one another in being limbless, but this ** conver- 

 gence," or " homoplasty," of form does not indicate any 

 relationship between them. 



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 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 ; thesecondary 

 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 in 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 



