100 GENERAL PRINCIPLES OF ZOOLOGY. 



other hand, functionless parts undergo a gradual decrease, which 

 finally leads to their disappearance. 



Change of Function of Organs. The two factors mentioned, 

 that the permanence of the tissues depends upon continued use, 

 and that usually several tissues enter into the structure of an 

 organ, are important for the understanding of the principle of 

 change of function which plays a prominent role in the meta- 

 morphosis of animal form. It may happen that an organ is 

 brought under changed conditions and no longer has an oppor- 

 tunity to function as before. In that case the functioning tissue, 

 from lack of use, gradually degenerates, but the organ may persist 

 by means of its accessory tissues if the new conditions make it 

 possible for one of them to attain to functional activity, and to 

 give the organ a new physiological character. 



Examples of Change of Function. A muscle, for example, may 

 become functionless from many causes. Should the muscle-tissue 

 disappear there are still left the accessory tissues, particularly 

 connective tissue permeated by blood-vessels; this may remain in- 

 tact and form a protecting band, a tendon, or fascia. We have 

 then, morphologically, the same organ, changed in its physio- 

 logical character; the muscle has undergone a change of function, 

 and has become a ligamentous band. The visceral arches of fishes 

 afford another example; these primarily are supports for the gills; 

 if now by the acquirement of terrestrial habits the gills be lost, 

 the visceral arches become functionless and correspondingly under- 

 go a partial degeneration; but a part persists by assuming a new 

 function, and forms the jaws, the hyoid bone, and the small bones 

 of the ear, 'which, in spite- of their quite different functions, are 

 morphologically the same structures as the gill-arches. 



Homology and Analogy. In the History of Zoology (page 14) 

 it was shown that comparative anatomy has caused a discrimina- 

 tion between homology or morphological equivalence, and analogy 

 or physiological equivalence, i.e., between organs which appear in 

 the same relative positions and relations, and organs which have 

 the same function. What we have here learned of the structure 

 of organs makes it evident that morphological and physiological 

 characters do not necessarily coincide, that morphologically similar 

 organs may have different functions, morphologically different 

 organs the same functions. 



Systems of Organs. Organs wholly identical, or, at least, 

 functioning in an equivalent manner, may occur in considerable 

 numbers in the same body. A man has many muscles, and many 



