HOMOLOGIES. 63 



Comparison of Organs. 



§ 53. 



The variations in organisation among the various larger and 

 smaller divisions of the Animal Kingdom are such as to lead us, at 

 first sight, to perceive the points of difference rather than those of 

 agreement. And this is more marked in proportion to the diver- 

 gence between the particular divisions compared. It is, however, 

 the business of Comparative Anatomy to follow out the changes in 

 the organisation, and to discover what is "similar" in the changed 

 and metamorphosed forms, however deeply hid it may be. An 

 organ may be " similar " to another in one of two ways. Either 

 in its functional relations, that is from a physiological point of 

 view ; or in its genetic and therefore anatomical relations, that is 

 from a morphological point of view. These two relations of an 

 organ must be kept well apart. The change of function in one and 

 the same organ, as well as the similarity in arrangement of organs 

 which are morphologically very different, compels us to ascribe a 

 subordinate value to physiological relations, when we are comparing 

 organs. The gills of a Fish, of a Crab, and of a Cephalopod, are 

 organs of respiration, and have many structural points in common ; 

 yet they are very different organs morphologically, as the relations of 

 each of the three to the whole organism shows. By insisting on 

 similarity of function, we bring together organs which are morpho- 

 logically different, and so turn aside from the object of Comparative 

 Anatomy. We distinguish, accordingly, physiological likeness, or 

 Analogy, from morphological hkeness, or Homology, and only 

 consider the proof of the latter as our task. 



The smaller the division to which the objects of comparison 

 belong, the more obvious is the homology. Homology therefore 

 corresponds to the hypothetical genetic relationship. In the more 

 or the less clear homology, we have the expression of the more or 

 less intimate degree of relationship. Blood-relationship becomes 

 dubious exactly in proportion as the proof of homologies is uncertain. 

 It is impossible therefore to say exactly how far homology extends 

 throughout the Animal Kingdom. As a matter of fact, numerous 

 investigations have discovered a larger number of homologous 

 arrangements even in otherwise divergent groups, and have thereby 

 extended the boundaries of homology further than was formerly 

 thought possible. 



In consequence of the existence of various possible modes of 

 morphological agreement, homology is divided into two primary 

 groups : General and Special Homology. 



