14 ELEMENTS OF P.IOLOGY. 



the essential conditions of vitality ; but from another point 

 of view it is wholly unnecessary. Light, namely, is neces- 

 sary for animated nature as a whole, but is by no means 

 essential to all living beings regarded as individuals. Many 

 animals spend a great part of their existence in total dark- 

 ness, and some pass their entire life without access to the 

 rays of the sun. Regarded, however, from a deeper point 

 of view, light is seen to be absolutely essential to life, since 

 vegetable life can only be carried on under the influence of 

 sun-force. All animals, as we shall subsequently see, are 

 dependent, mediately or immediately, upon plants for their 

 food ; since plants alone possess th» power of building up 

 organic compounds out of inorganic materials. Plants, 

 however, can perform this feat of vital chemistry only when 

 supplied with the light-giving and chemical rays of the sun, 

 so that light is an absolute prerequisite for life. The im- 

 portance of light as one of the conditions of life, will, how- 

 ever, be spoken of at greater length in treating of the food 

 of animals and plants, and the distribution of animal life at 

 great depths in the ocean. 



d. Air. — The presence of atmospheric air, or rather of 

 free oxygen, appears to be essential to animal life, and a 

 supply of oxygen may therefore be regarded as one of the 

 extrinsic conditions of vitality. It would seem, however, 

 that certain low vegetable organisms (vibriones and bacte- 

 ria) flourish in an atmosphere of carbonic acid ; so that free 

 oxygen cannot be looked upon as being an indispensable 

 requisite of life. 



e. Temperature. — In a general way, the higher manifesta- 

 tions of life are only possible between certain limited ranges 

 of temperature, which may be stated as varying from near 

 the freezing-point to 120'^ or 130° Fahrenheit. Some of the 

 lower forms of life, however, can unquestionably endure 

 temperatures much more extreme than these ; and it would 

 appear that life in its lowest grades is not impossible at tem- 

 peratures considerably below the freezing-point, and rising 



