CHANGE OF FUNCTION. 29 



Similarly, gills are analogous but not homologous with lungs. 

 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-limbs, and both are organs of flight. 



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 are respiratory as well as loco- 

 motor, and part of the food-canal of Tunicates and 

 Amphiqxus 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 dimin- 

 ution 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 modifi- 

 cation 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 birth-robe 

 ^(chiefly respiratory) around the embryo, and in most 

 Mammals forms part of the placenta which effects nutritive 

 connection between offspring and mother. The stalk of this 

 allantois forms the urinary bladder of Mammals, and of those 

 Reptiles which have one. In illustration it is sometimes 

 said that the swim-bladders of Fishes, especially of the 

 Dipnoi, represent the lungs of higher Vertebrates, but there 

 are several objections to this statement. 



" Substitution of Organs." — The idea of several changes 

 of function in the evolution of some organs, suggests another 

 of not less importance which has recently been emphasised 



