IXTRA-SELECTION OR SELECTION AMONG TISSUES 245 



muscle or a giand be cut through, the organ concerned begins to 

 degenerate and to lose its normal structure to a greater or less degree. 

 Sensory nerves also degenerate in their peripheral part when they are 

 cut through. In such cases there may be no alteration either in the 

 nutritive mechanism or in the blood-vessels, &c., but the functional 

 stimulus— in the case of the muscle, the stimulus from the will — no 

 longer affects the organ, and its metabolism is so much lowered in 

 consequence that it begins to degenerate. 



When we admit that the fit adaptation of the organism, as far 

 as we understand it, must depend upon processes of selection, we 

 may refer this ' functional adaptation ' also to primitive processes of 

 selection, which prevailed at the very beginning of life upon our 

 earth, and represented, so to speak, the first adaptation that was 

 established, but we can say nothing with certainty in regard to this 

 matter as long as we do not understand the essence of assimilation. 

 It is conceivable, however, that a 2}'^"i''niciry adaptiveness may have 

 arisen, so to speak, abruptly, through a concurrence of favourable cir- 

 cumstances, as we shall endeavour to show later on when we discuss the 

 beo-innino-s of life. 



Even although we cannot lay bare the primary roots of ' func- 

 tional adaptation ' we can gain from the fact itself very valuable 

 insight into phenomena which would otherwise be unintelligible and 

 mysterious: tlie perfectly adapted dructure of many tissues and their 

 ■poiver of adapjtation to clmngcd conditions. In this lies, in the main, 

 the advance in our knowledge which is due to Roux's Kampf der 

 Theile. 



If a number of embryonic cells of different capacity, say A, B, 

 and C, be affected by different kinds of functional stimuli, a, h, and c, 

 those cells will grow most rapidly which are most frequently affected 

 by the stimulus appropriate to them. The proportion in which the cells 

 A, B, and C will ultimately be present in the tissues will depend upon the 

 frequency with which the stimuli a, h, and c act upon the tissue. But 

 the tissue will be still more precisely determined as to its structure if the 

 three kinds of stimuli affect the cell-mass, not uniformly all over, but 

 only at certain spots, or along particular paths, one in this, the other 

 in that. Thus the cells A will predominate over the cells B and G at 

 all the places which are most frequently affected by the stimulus a, 

 the cells B in the sphere of the stimulus h, and the cells C in that of 

 the stimulus c ; there they will increase most rapidly and so crowd 

 out the other kinds of cells, and thus a spatial arrangement will be 

 established within the tissue, a ' structure ' which corresponds and 

 is well adapted to its end. This is what Roux deduced from his 



