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 

 Amoeba, up to the complicated instinctive and rational acts 

 of man. 



