TYPES OF CONSTRUCTION IN THE ANIMAL KINGDOM. 22/ 



Cephalization one of the Expressions of the General Law of 

 Differentiation. — Cephalization nia\' be reg^arded as but one of 

 the expressions of the general princi[)le of differentiation. 

 Differentiation concerns the whole organism, because increase 

 in the specialization of function of one organ always involves 

 the provision, through the activity of other parts, for the 

 supply to that organ of resources which it fails to supply to 

 itself. 



Meaning of Homology and Homologous Parts. — When animals 

 are compared there are some terms which are applied to the 

 relationship noted in the parts compared ; a few of the terms 

 are the following: 



Homology and Jiouiologoiis are applied to the organs or 

 parts of different organisms which correspond in type of 

 structure. Thus the secondary joints of the appendages of 

 Arthropods are homologous parts, and one appendage may be 

 used as a swimmer or a claw, another as a mandible, and 

 therefore be constructed in different form ; but the parts, 

 although of different form, are said to be homologous, be- 

 cause modifications of the same element of differentiation 

 (see Fig. 50). 



Another example is the case of the forearm of a bird 

 and the forearm of a bear. When the bones are compared 

 they are found to possess corresponding parts — a shoulder- 

 blade, a humerus, a radius, an ulna, a carpus, metacarpus, 

 and finger-bones. Although the arm in one case is adapted 

 to the function of flying in the air, in the other to walking on 

 the ground, and the shape of each bone is different, the 

 several parts are homologous, because bearing the same rela- 

 tion to the structure of the whole, and representing the same 

 typical part of the primitive structure. 



Analogy and Analogous Parts. — Analogy is used in a different 

 sense. Two parts or organs of different animals are said to 

 be analogous when the likeness has to do with the functions 

 or adapted usage of the parts; and not to either the mor- 

 phology, or the relationship to other parts of the organic 

 structure of the animal. For instance, the leg of a fly and 

 the leg of a mouse serve the same function — walking or loco- 

 motion, but they differ morphologically, i.e., in form; they 



