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182 SCIENCE PROGRESS 



Each living cell is a machine ; it breathes, taking in oxygen ; 

 it feeds, and the food is burnt by the oxygen to chemically 

 simpler bodies. The living cell, like the gas engine, can tap 

 the stores of chemical energy — and, like the gas engine, it is 

 an internal combustion engine. Now in a power station where 

 electricity is being produced to run a score of trams, there 

 is a steady hum or drone, the varying pitch of which marks 

 the speed of the engine. To the engineer in charge, from 

 long habit, that varying sound speaks of events happening in 

 remote parts of the system. A glance at the clock, and he 

 will tell you that the sound is falling because the engine is 

 adjusting itself to the increased load due to such and such 

 a tram breasting such and such a hill. In the same way, 

 watching the movement of a living cell under the microscope, 

 if we were sufficiently skilled we could refer the continual 

 change in the rate and direction of its movement to temporary 

 inequalities of temperature, of lighting or of chemical com- 

 position, etc., in the water in which it lives. If we resort to 

 experiment, the effects are obvious : an electric shock causes 

 the irregular Amoeba to come to rest as a sphere, a trace of 

 acid slows its movements, of alkali accelerates them. 



These things — the electric shock, the acid, or the alkali — are 

 what the biologist calls " stimuli," and by varying their nature 

 or intensity he can control the activity of living matter to a 

 very remarkable extent. 



Let us return for a moment to the Amoeba. We watch it 

 crawling amid sand, fragments of decayed leaves, and living 

 diatoms, and we notice that of the particles which it eats 

 some are nutritious food, some are innutritious and absolutely 

 useless. But we also notice that there is a decided balance 

 in favour of the nutritious particle. Like Autolycus, it is a 

 picker-up of unconsidered trifles, guided by a decided prefer- 

 ence for things useful to itself. Therefore, the tiny animal 

 manifests discrimination or choice — imperfect, no doubt, but 

 clearly recognisable. And the choice is beneficial ; it contains 

 an element of purpose. 



Watch an Amoeba long enough, and it will be seen to divide 

 into two, and these again into two, so forming successive 

 generations the individuals of which resemble one another. 

 This labile, creeping fragment of jelly has recognisable form, 

 and zoologists classify the various forms in so many species, 



