152 DAYS OUT OF DOORS. 



few, the sound is deep ; as they increase in number, it be- 

 comes shriller and shriller; but when they reach forty 

 thousand in a second they cease to be audible. Light is 

 the effect produced on us when waves of light strike on 

 the eye. When four hundred millions of millions of vi- 

 brations of ether strike the retina in a second, they pro- 

 duce red, and as the number increases the color passes 

 into orange, then yellow, green, blue, and violet. But be- 

 tween forty thousand vibrations in a second and four hun- 

 dred millions of millions we have no organ of sense capa- 

 ble of receiving the impression. Yet between these lim- 

 its any number of sensations may exist. We have five 

 senses, and sometimes fancy that no others are possible. 

 But it is obvious that we can not measure the infinite by 

 our own narrow limitations. 



" Moreover, looking at the question from the other side, 

 we find in animals complex organs of sense, richly sup- 

 plied with nerves, but the function of which we are as 

 yet powerless to explain. There may be fifty other senses 

 as different from ours as sound is from sight ; and even 

 within the boundaries of our own senses, there may be 

 endless sounds which we can not hear, and colors, as dif- 

 ferent as red from green, of which we have no conception. 

 These and a thousand other questions remain for solution. 

 The familiar world which surrounds us may be a totally 

 different place to other animals. To them it may be full 

 of music which we can not hear, of color which we can 

 not see, of sensations which we can not conceive. To 

 place stuffed birds and beasts in glass cages, to arrange 

 insects in cabinets, and dried plants in drawers, is merely 

 the drudgery and preliminary of study; to watch their 

 habits, to understand their relations to one another, to 

 study their instincts and intelligence, to ascertain their 

 adaptations and their relations to the forces of nature, to 

 realize what the world appears to them — these constitute. 



