244 THE EVOLUTION THEORY 



been an open, if not quite conscious, secret, that ' functional stimulus 

 strengthens the organ,' that is to say, that an organ increases through 

 its own specific activity. Up till that time it had been believed that 

 it was merely the increased flow of blood that caused the increase in 

 the size of a much-used part. Roux showed that there is a ' quantitative 

 self-regulation of the organ according to the strength of the stimulus 

 supplied to it ' ; that the stimulated organ, that is, the organ which is 

 performing its normal function, may, in spite of the increased breaking 

 down or combustion (dissimilation), assimilate all the more rapidly; 

 that its used-up material is ' over-compensated,' and that therefore it 

 grows. He called this the ' trophic ' or nutritive effect of the stimulus, 

 and in terms of this he explained the increase and the heightened 

 functional capacity of the much -used organ. Conversely, he referred 

 the decrease of a disused organ to ' functional atrophy,' which sets in 

 when there is a deficient compensation for the substance used up in 

 the metabolism. 



But if we press for deeper analysis, we must ask : ' On what does 

 this trophic effect of functional stimulus depend?' Roux could not 

 answer this question when he wrote, nor can we do so now, as Plate 

 has justly emphasized. We are here face to face with the fundamental 

 phenomenon of life, metabolism ; and, since we do not understand the 

 causes of this, we are not in a position to say why it varies in this 

 way or in that according to the 'stimulus.' But the fact itself is 

 certain that the organs respond up to a certain point to the claims 

 made upon them ; they increase in proportion as they function more 

 frequently or more vigorously, they are able to respond to increased 

 functional demands, and this Roux has called ' functional adaptation.' 

 As an animal adapts itself to the claims of the conditions of its life, 

 for instance, by taking on a green or a brown protective colour 

 according as it lives on green or brown parts of plants, so the 

 individual organ adapts itself to the strength of the stimulus which 

 impels it to function, and increases or decreases in proportion to 

 it. If one kidney in Man degenerate, or be surgically removed, the 

 other begins to grow, and goes on increasing until it has reached 

 nearly twice its former size. The specific stimulus which is brought 

 to bear upon it by the urea contained in the blood, and which forces 

 it to grow, is twice as great in the absence of the other kidney, and 

 therefore the remaining kidney grows in response to the increased 

 stimulus and its ' trophic efiect ' until its increase in size has reduced 

 the functional intensity to the normal proportion. 



Adaptation of an organ in the opposite direction takes place 

 when the function diminishes or ceases. If a nerve supplying a 



