122 THE EVOLUTION THEORY 



in regard to the composition and activity of the determinants in 

 the £erm. 



In the meantime we must be content with the knowledge that, 

 on the determinant hypothesis, the disappearance of organs which have 

 become useless may be regarded as a process of intra-selection going 

 on between the 'primary constituents' (Anlagen) of the germ, and 

 depending on the same principle of the ' struggle of parts ' which 

 William Roux introduced into science with such brilliant results. 

 If a struggle for food and space actually takes place, then every 

 passive weakening must lead to a permanent condition of weakness 

 and a lasting and irretrievable diminution in tlie size and strength 

 of tlie primary constituent concerned, unless personal selection 

 intervenes, and choosing out the strongest among these weakened 

 primary constituents, raises them again to their former level. But 

 this never happens when the organ has become useless. 



This explains why not only parts with active function, like limbs, 

 muscles, tendons, nerves, and glands, disappear when they cease to 

 function, but also passive parts like the colouring of the external 

 surfaces of animals, the lifeless skeletal parts of Arthropods and the 

 exact adaptation of their thickness to the dwindling function, the 

 disappearance of superfluous wing-veins, and of the hard chitinous 

 covering of the abdomen when it is concealed in a protecting house, 

 as in the case of hermit-crabs, Phryganida?, and Psychidse. Here too 

 we find a sufiicient explanation of the fact that parts whicli have 

 become functionless, such as the wings of ants, can disappear even in 

 the case of sterile workers. 



The principle of germinal selection, however, can only be under- 

 stood in its full significance if we take the positive aspect also into 

 consideration. We had reached the conclusion that because of the 

 fluctuations of the food-supply one set of the homologous determinants 

 represented in the various ids may vary in a minus direction, and 

 another set in a plus direction, and that this direction will be adhered 

 to as long as no intra-germinal obstacles come in the way. As long 

 as this does not happen the determinant concerned will pursue the 

 path of variation it has once struck out, and indeed the tendency 

 will be strengthened, because every passive variation, upwards or 

 downwards, results in a streno-thenino- or weakenino- of the deter- 

 iiiinant's power of assimilation. 



Let us take a case of positive variation of the determinants of an 

 organ N, which would be more useful to the species if it were more 

 liighly developed than it had previously been. The variation in an 



