12O GENERAL PRINCIPLES OF ZOOLOGY. 



Examples of Change of Function. A muscle, for 

 example, may become functionless from many different 

 causes. Should the muscle-tissue disappear there is still left 

 the sum of the accessory tissues, particularly, connective 

 tissue permeated by blood-vessels; this may remain intact 

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

 then, morphologically, the same organ, only that its physio- 

 logical character has changed ; the muscle has undergone a 

 change of function, and has become a ligamentous cord. 

 The visceral arches of fishes are another example ; these in 

 their primary significance are supports for the gills; if now 

 by the acquirement of terrestrial habits the gills are lost, 

 the visceral arches become functionless and correspondingly 

 undergo a partial degeneration ; but a portion maintains 

 itself 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 1 6) it was shown that Comparative Anatomy has 

 caused a discrimination between homology or morphological 

 equivalence, and analogy or physiological equivalence, i.e., 

 between organs which appear in the same relative positions 

 and the same mode of connection, and organs which have 

 the same function. What we have here learned to recog- 

 nize in the structure of organs makes it evident that 

 morphological and physiological characters do not nec- 

 essarily 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 organs which carry on digestion. 

 Hence we may group all organs which in the body have 

 equivalent or similar functions into an ideal higher unit, 

 and speak of systems of organs. In all we recognize nine 

 such systems: (i) skeletal, (2) digestive, (3) respiratory, 



