13 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. We 

 notice that the under lip of a worm is comparable to the foot 

 of a mollusk, though the functions of these j)arts 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 fore- 

 arms of man. 



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 gvasj^s, 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 

 Ammha, up to the complicated instinctive and rational acts 

 of man. 



