100 THE DATA OF BIOLOGY. 



the abstract two different aspects — the objective and the sub- 

 jective. Practically, however, the objective, which deals 

 with these mental modifications as exhibited in the changing 

 habits and abilities of successive generations of creatures, is 

 the only one that admits of scientific investigation ; since the 

 corresponding alterations in consciousness, cannot be imme- 

 diately known to any but the subjects of them. Evidently, 

 convenience requires us to class this part of Psychology along 

 wfith the other parts, in a distinct sub-science. 



Light is thrown on functions, as well as on structures, 

 by comparing organisms of different kinds. Comparative 

 Physiology and Comparative Psychology, are the names 

 given to those collections of facts respecting the homologies 

 and analogies, bodily and mental, that are brought to light by 

 this kind of inquiry. These classified observations concern- 

 ing likenesses and differences of functions, are helpers to 

 interpret functions in their essential natures and relations. 

 Hence Comparative Physiology and Comparative Psychology 

 are names of methods, rather than names of true sub-divisions 

 of Biology. 



Here, however, as before, the comparison of special truths, 

 besides facilitating their interpretation, brings to light certain 

 general truths. Contrasting bodily and mental functions as 

 exhibited in various orders of organisms, shows that there 

 exists, more or less extensively, a community of processes 

 and methods. Hence result two groups of abstract propo- 

 sitions, constituting General Physiology and General Psy- 

 chology. 



§ 40. In these various divisions and sub-divisions of the 

 first two great departments of Biology, the phenomena of 

 Structure are considered separately from the phenomena of 

 Function, so far as separate treatment of them is possible. 

 The third great department of Biology deals with them in 

 their necessary connexions. It comprehends the determin- 



