PHENOMENA AND CONDITIONS OF LIFE 31 



organisms, the bacteria, there are many for which life is only 

 possible if free oxygen is completely excluded indeed, they 

 die when brought into contact with it. They are known as 

 anaerobic bacteria. It is, however, not improbable that even 

 anaerobic microbes use oxygen which they obtain from the 

 various stable compounds of their nutrient media, and that only 

 a liberal supply of free oxygen is fatal to their existence. 



Another of the fundamental conditions of life is water. 

 Whatever animal or plant we may examine, we shall always find 

 that their cell-protoplasm contains water. Indeed, water is the 

 chief component of all organisms. The human body contains 

 about sixty parts of it, the tissues of the muscles from seventy-five 

 to eighty. That many plants and parts of plants, in particular 

 fruits, consist largely of water is well known. Many denizens of 

 the sea, especially the jellyfishes and ctenophores, contain two 

 parts of solid matter and ninety- eight of water. It is remarkable 

 how under such conditions a structure can be maintained. One 

 may frequently see on the sea-shore a large brilliantly coloured 

 medusa, stranded high and dry, in a short time disappear before 

 one's very eyes, leaving nothing behind but a little heap of 

 crumpled membrane. 



Warmth is another conditio sine qua non. The limits of 

 temperature between which active life is possible are very narrow, 

 for great heat and great cold are equally hostile to it. Though 

 many organisms may, owing to the warmth of their bodies, exist 

 for considerable periods in a temperature far below zero, all 

 active life is doomed to cease as soon as the interior of the body 

 has cooled down to the freezing-point of water, for only when 

 water can retain its liquid form nutrition and respiration are 

 possible. In the higher organisms life ceases in temperatures 

 which coagulate albumen. Between these two extremes the life- 

 functions of all organisms operate, though the optimum of well- 

 being is, in the various species of plant or animal, subject to 

 considerable variations of temperature. 



If we observe a protozoon, the minute Amoeba Umax, in a 

 temperature of + C. we shall see that it rests suspended in its 

 drop of water, contracted into a globule, devoid of any action. 

 It is said to be in a state of * chill-coma.' But if we now 

 increase the temperature we observe that at 2 to 5 C. a slight 



