HOMOLOGIES. 65 



the alimentary canal as homologous ; while in the Vertebrata we 

 can confidently assert that even such unimportant structures as 

 the casca of the intestine from the Amphibia onwards are homo- 

 logous. The homologies of the parts of the skeleton, which are thu 

 organs that have been investigated with the greatest exactness, are 

 those which can be most definitely recognised. It is a large part 

 of the main task of Comparative Anatomy to prove special homo- 

 logies. 



Special homology must be again separated into subdivisions, 

 according as the organs dealt with are essentially unchanged in 

 their morphological characters, or are altered by the addition or 

 removal of parts. I therefore distinguish — 



1) Complete Homology, when the organ referred to is 

 unchanged in position and connection, and is still perfect however 

 much modified in form, size, and various other points. This kind 

 of homology is generally found within the limits of small divisions, 

 less often in larger ones. For example, the bones of the upper 

 arm from the Amphibia to the Mammalia, the heart of the Amphibia 

 and Reptilia, etc., exhibit complete homology. 



2) Incomplete Homology. This consists herein, that an 

 organ which is otherwise completely homologous with another, has 

 other parts which are wanting in the latter added to it, or conversely, 

 when an organ is wanting in some essential part in comparison with 

 another organ. The heart of the Vertebrata may serve as an 

 example. The organ is homologous throughout the division from 

 the Cyclostomi onwards, but the homology is incomplete; for in 

 Fishes a part, the venous sinus, which in the higher divisions is 

 taken into the heart, and which in the Mammalia is absorbed into 

 the right auricle, lies outside the heart. The homology between the 

 heart of the Fish and of the Mammal is consequently incomplete 

 owing to addition. In another case it may be incomplete owing 

 to diminution. The reverse of the previous case may serve as an 

 example, were it allowable to regard the heart of the fish as a 

 reduced one. An example is presented by the pectoral fins of 

 fishes. The skeleton of this organ in the Ganoidei or Teleostei 

 is, owing to reduction, incompletely homologous with that of the 

 Selachi. Parts have in this case disappeared which did primitively 

 belong to the organ, just as in the former case parts, which although 

 they were primitively present did not belong to the oi'gan, were 

 added to it. 



Systematic Classification of the Animal Kingdom. 



§ 56. 



In the general organisation of every animal we recognise a 

 number of arrangements which it has in common with a greater or 

 less number of other animals. These relations are partly of a more 



p 



