GERMINAL SELECTION 123 



upward direction is at first purely passive, having arisen from 

 fluctuations in the food-supply, but it soon becomes active, since 

 the determinants that have become stronger will have a stronger 

 affinity for food and will attract more and more of the availa))le 

 supply. The increased food-stream is thus maintained, and its gradual 

 result is such a strengthening (jf the determinants in the course 

 of generations of germ-cells, that the parts controlled l)y these 

 determinants — the determinates— nmst enter on a path of plus- 

 variations. If to this there be added j^ersonal selection, either natural 

 or artificial, any fluctuations of this primary constituent towards the 

 minus side will be effectually prevented, the direction of variation 

 will remain positive, and the continued intervention of personal selection 

 may raise its development to its possible maximum, that is, so far 

 that further development in the same direction would not make 

 for greater fitness, and personal selection must call a halt. This will 

 always happen as soon as further increase of the organ would be 

 prejudicial to the living power of the whole, and when the harmony 

 of the bodily parts would thereby be permanently disturbed. 



That variation in an upward direction reall}^ can persist for 

 a long time is shown by artificial selection as practised by Man in 

 regard to his domesticated animals and cultivated plants. At first 

 general variability, or at least variability in many directions, sets in 

 as a result of the greatly altered conditions of life ; the ordinary 

 fluctuations of the determinants are intensified by the greater fluctua- 

 tions in the nutritive stream, and it becomes possible for Man 

 consciously or unconsciously to select for breeding whatever he 

 prefers among the chance variations that arise in individual parts 

 or in whole complexes of parts, and he may thus give rise to 

 a long-continued, often apparently unlimited, augmentation of varia- 

 tions in the same direction, although he cannot exercise any direct 

 influence upon the germ-plasm or its determinants. When a deter- 

 minant has assumed a certain variation-direction it will follow it 

 up of itself, and selection can do nothing more than secure it a free 

 course by setting aside variations in other directions by means of 

 the elimination of those that exhibit them. 



That artificial selection can cause the increase of a part has long 

 been established, but in what way this is possible, and how it can 

 be theoretically explained has hitherto been xvvy obscure, for even 

 if we take the favourable case that l)oth parents possess the desired 

 variation, it cannot be supposed that the characters of the parents are, 

 so to speak, added together in the child ; all we can say is that 

 the probability that the children will also exhibit the character m 



