THE GENERAL CONDITIONS OF LIFE 279 



part their life-work. They require for their metabolism ferrous 

 carbonate, which is dissolved in the water. They absorb this and 

 oxidise it into ferric carbonate, which they give off to the 

 outside. The excreted ferric carbonate in time passes over 

 into simple ferric oxide, which is insoluble, and forms a yellowish- 

 brown precipitate upon the gelatinous covering excreted by the 

 bacteria, in which their bodies lie. If the iron-bacteria be culti- 

 vated without ferrous carbonate, their vital phenomena become 

 gradually feebler and finally come to a complete standstill. Hence 

 the presence of this substance belongs among the conditions of 

 life of these remarkable microbes. 



These examples suffice to show how peculiar may be the special 

 conditions of life among different organisms as regards food. 

 This is not the place for their further consideration ; they belong 

 to the province of special physiology. 



2. Water 



Living substance is liquid. It is necessary to remember this 

 fundamental physical property. The liquid jelly-like condition of 

 living substance is due to the water that it contains, which fact 

 can be proved easily by evaporating the water. Only liquid, not 

 solid masses, only substances that contain water can be living, for 

 only with the liquid state is metabolism compatible. Hence in the 

 organism all substances that are solid and hard, such as the con- 

 nective tissues of the teeth and the bones, are not living. 

 Similarly, vital activity is lessened along with the withdrawal 

 of water. In dried Rotifera and Tardigrada, and in dried seeds, 

 no vital phenomena can be perceived. Life begins to manifest 

 itself only when the seeds are made to swell by the addition of 

 water, only when the substance of their cells becomes again 

 liquid. Water, therefore, belongs to the general conditions of 

 life. This conclusion is very simple and clear. But there are 

 cases where, even in places of the greatest drought, organic life 

 continually exists. In spite of their dryness the waste, burning 

 deserts of Arabia and Africa, which present to the traveller most 

 powerfully impressive pictures of eternal lifelessness, and whose 

 sands are moistened scarcely once in a year by showers of rain, 

 harbour manifold varieties of animals and plants. This apparent 

 exception depends upon the fact that all desert-organisms are 

 peculiarly adapted to life in long drought, and they manage ex- 

 tremely frugally and economically with the little water that comes 

 to hand at long intervals of time. One is astonished in the driest 

 desert to come upon green plants that contain abundant juices, 

 plants (Mesembryanthemum crystallinum) beset over and over with 

 cells, which harbour such quantities of clear water that the latter 



