12 ZOOLOGY. 
mental laws of evolution. On the other hand, pure Dar- 
winism—viz., natural selection—accounts rather for the 
preservation than the origination of the forms of life. 
Analogy and Homology.—When we study the Inverte- 
brates alone we see that it is often easy to trace a general 
identity in form between the more important parts. The 
parts of the sting of a bee are originally like the feet or jaws 
of this insect, though the functions of these parts may be 
quite unlike; these are therefore examples of a general 
identity in structure or homology between two organs. A 
closer homology implies a more apparent identity of form, 
as seen in the resemblance in structure of the fore-limbs of 
a whale and a seal, or the pectoral fins of fishes and the 
arms of man, or the wing of a bird and the human arm. 
Analogy implies a dissimilarity of structure of two organs 
with identity in use, as the wing of an insect and of a bird ; 
the leg of an insect and the leg of a frog; the gill of a 
worm and the gill of a fish. 
Homology implies blood-relationship ; analogy repudiates 
any common origin of the organs, however physiologically 
alike. The most general homologies are those existing in 
organs belonging to animals of different branches ; the most 
special between those of the same orders and minor groups. 
Thus it is fundamentally a question of near or remote con- 
sanguinity. 
Physiology treats of the mode in which organs do their 
work ; or, in other words, of the functions of different or- 
gans. Thus the hand grasps, the fins of a fish are its swim- 
ming organs ; the function of the nose is to smell, of the 
liver to secrete bile, of the ovary to secrete protoplasm 
which forms eggs. 
Psychology is the study of the instincts and reasoning 
powers of animals; how they act when certain parts are 
irritated ; so that while this term is generally applied to 
man alone, Comparative Psychology deals both with the 
simplest automatic acts and the whole series of psychic pro- 
cesses—from those exercised by the Protozoans, such as 
Ameba, up to the complicated instinctive and rational acts 
of man. 
