210 THE STUDY OF ANIMAL LIFE CHAP. 



take a particular case, the skull of a carnivorous marsupial, 

 such as a Tasmanian Devil, shows many convergent or 

 homoplastic resemblances to the skull of one of the 

 Carnivora, such as a dog. 



8. Change of Function. Organs are not mechanisms 

 rigidly adapted for only one purpose. In many cases 

 they have a main function and several subsidiary func- 

 tions, and changes may take place in organs by the occa- 

 sional predominance of a subsidiary function over the 

 original primary one. Dr. Anton Dohrn, the founder 

 of the Naples Zoological Station, especially emphasised 

 the idea of function-change. He wrote :- 



" 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 acces- 

 sion of a secondary function changes the total function ; the 

 secondary 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 the 

 Torpedo and some other fishes that give a shock, the 

 electric changes in the modified muscular tissue com- 

 posing the organ have become more important than the 

 contractility. The swim- or air-bladder which grows 

 out dorsally from the food-canal of most fishes, seems 

 usually to be a hydrostatic organ ; in a few cases it 

 helps slightly in respiration, but in the double-breathing 

 mud-fishes or Dipnoi it has become a genuine lung. 

 An unimportant (allantoic) bladder at the hind end of 

 the gut in frogs, is represented in the embryos of 

 reptiles and birds by a very important respiratory 

 (and sometimes yolk-absorbing) birthrobe, and in almost 

 all mammals by part of the placenta which unites mother 

 and unborn offspring. 



We seem to have here a disclosure of one of the methods 

 of organic evolution that apparently novel things are 

 often rehabilitations of very old structures. The ele- 

 phant's trunk was a novelty in its day (though it evolved 

 gradually enough), but it is, after all, little more than 



