THE NATURE OF ANIMALITY. 211 



14. As the facts of human psychology are attained by a 

 process of self-examination, it is evident that we can only 

 investigate comparative psychology by an indirect method. 

 Nevertheless, as we can compare the combined instinctive and 

 rational elements of our own human economy, so we may with 

 confidence conduct our indirect comparative psychological 

 investigations under the control of our own experience. 



15. There can be no question as to the existence in the 

 animal of a principle allied to our own human consciousness. 

 This is admitted by common consent. Every unbiassed observer 

 who has studied the actions of the various forms of animals, 

 from the protozoon upwards, must feel impressed by the mani- 

 festations presented of an ascending series of forms of con- 

 sciousness ; a series co-ordinate with the series of structural 

 forms in which their presence is manifested. 



16. These manifestations evince in various degrees of 

 distinctness, as we ascend in the series, three fundamental 

 conditions or states of consciousness. 



1st. As the consciousness is manifested in sensation and 

 perception. 



2d. As it is manifested in the appetites, emotions, passions, 

 social impulses, and special habits ; and 



3d. As it manifests itself in the determining power or will. 



17. In attempting to assign a precise value or meaning to 

 the terms sensation, perception, appetite, emotion, passion, 

 will, when applied to forms of consciousness in an animal, we 

 can only proceed by the indirect method. 



18. The sense of vision, as manifested in the animal. 

 affords a satisfactory illustration of this indirect method of 

 inquiry. The organ of vision is met with very low in the 

 animal Beries. 



L9. Bowevei varied the structure of the eye may lie, it 

 is always constructed on optical principles. The refractive 



ni'du of llir eye aiv -u ;u lan-i'd and n >n M ruded as to trails- 



