14 ELEMENTS OF BIOLOGY. 



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 the 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 



