2 INTRODUCTION. 



perform a similar function are termed analogotis. In many 

 cases one part may be both analogous and homologous 

 with another. 



Again, if we take a number of structural characters in 

 an organism, these can be divided into inherited d^Vidi acquired. 

 In the former case, the structure is such because of the 

 tendency in all organisms to resemble their parents ; in the 

 latter it is such because of the capacity of an organism to 

 adapt itself to its surroundings. 



Hence the study of an organism resolves itself into the 

 following: — 



1. Morphology or study of form. <;'tt^^,„^, '^ 



■' ^"rllSIOLOGY. 



( Homology = similarity in structure.) 



2. Physiology or study of function. 



(Analogy = similarity in function.) 



3. Embryology or study of the early history of an organism. 



We now have to consider the relationship of an organism 

 to other organisms. The comparison of structure, function 

 and development can obviously be called Comparative 

 Anatomy, Comparative Physiology, and Comparative Em- 

 bryology respectively, but there are one or two other points 

 to notice. 



If we take two closely allied organisms their structure 

 will show a certain degree of similarity or homology. 

 This similarity must in each case be due to one of two 

 causes. It is either due to the fact that the two organisms 

 are descended from a common ancestor, and therefore 

 inherited^ or it is due to the two organisms having lived in 

 a similar environment, and thus acquired. The form of 

 homology in the first instance is termed homogeny. and 

 that in the latter homoplasy.'^ 



Two brothers owe their similarity to homogene : or 

 inherited homology, and two sailors owe their similarity in 

 uniform, gait and habits to homoplastic or acquired 

 homology. The distinction is clear when such a crude 

 example is given, but, if we assume the sailors to be 

 brothers, one would be in great doubt whether to ascribe 

 some similarities to one or to the other kind of homology. 



* The terms Palingenetic and Ccenogenetic are often used in much the 

 same sense as homogeneti and homoplastic. 



