^IQ THE EVOLUTION THEORY 



thus only chance variations which were inherited through panmixia 

 and gradually ditfused over the whole species, how could it come 

 about that all the variations were in the direction ot smaller size^ 

 Yet this is obviously the case. Why should no variations in the 

 direction of larger size occur 1 And if this were so, why should 

 a useless organ not be maintained at its original size, if it be admitted 

 that an increase in size would be prevented by natural selection 1 

 But this never occurs, and diminution in size is so absolutely the 

 rule that the idea of a ' vestigial or rudimentary ' organ suggests 

 a 'small organ' almost more than an 'imperfect' one. 



There must then be something else at work which causes the 



minus- variations in a disused organ to preponderate persistently and 

 permanently over the plus-variations, and this something can He 

 nowhere else than where the roots of all hereditary variations are to 

 be found-in the germ-plasm. This train of thought leads us to the 

 discovery of a process which we must call selection between the 

 elements of the germ-plasm, or, as I have named it shortly, Germinal 



Selection. , 



If the substance of the germ-plasm is-as we assumed-composed 

 of heterogeneous living particles, which have dissimilar roles m the 

 building up of the organism, there must of necessity be among them 

 a definite labile state of equilibrium, which cannot be disturbed 

 without modifying in some way the structure of the orgamsm itselt 

 which arises from the germ-plasm. But if our further view be 

 correct, that these individual and different living umts of the germ- 

 plasm are ' determinants,' that is, are the primary constituents o 

 particular parts of the organism, in the sense that these parts could 

 not arise if their determinants were absent from the germ-plasm, and 

 that they would be different if the deterniinants were differently 

 composed, we can draw far-reaching deductions. 



It is true that we cannot learn anytJdng directly m regard to 



the intimate structure of the germ-plasm, and even in regard to the 



vital processes going on within it we can only guess a very little 



but so much we may say-that its living parts are nourished, and 



that they multiply. But it follows from this that nourishmem in 



a dissolved state must penetrate between its vital particles, and that 



whether the determinants grow, and at what rate they do so, depends 



mainly on the amount of nourishment which reaches them. As long 



as the germ-cells multiply by division the determinants have no other 



function but to grow; a part of their substance undergoes oxidation 



and thereby yields the supply of energy necessary to assimilation, 



that is, to the formation of new living substance. 



