2 LIVLMULYYUE Liev. 
perform a similar function are termed analogous. 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 iwherited and acguzred. 
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 :— 
ANATOMY. 
HIsTo.ocy. 
(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 acguived. The form of 
homology in the first instance is termed homogeny, and 
that in the latter homoplasy.* 
Two brothers owe their similarity to homogenetic 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. 
1. MORPHOLOGY or study of form. << 
* The terms Palingenetic and Canogenetéc are often used in much the 
same sense as homogeneti and homoplastic. 
