GERMINAL SELECTION 129 



brium there may occasionally be marked fluctuations in the nutritive 

 stream, and thus more than usually pronounced variations of the deter- 

 minants affected by it will occur. These yield the material for new 

 adaptations if they are in the direction of fitness, or they are eliminated 

 either by the chances of reducing division or by personal selection if 

 new adaptations are not required. 



The old-established hereditary equilibrium of the germ-plasm 

 must be most easily disturbed when the species is in some way 

 brought into new conditions of existence, as, for instance, when plants 

 or animals are domesticated, and when in consequence, as we have 

 already assumed, the nutritive currents within the id gradually alter, 

 quantitatively and qualitatively; and on this account alone certain 

 kinds of determinants are favoured, while others are at a disadvantao;e. 

 In this way there arises the intensified general variability of domes- 

 ticated animals and cultivated plants which has been known since the 

 time of Darwin. Something analogous to this must occur in natural 

 conditions, though more slowly, when a species is subjected to a 

 change of climatic conditions, but we shall discuss this later on in 

 more detail. 



We have thus arrived at the idea that the slight variations of the 

 determinants may be counteracted whether they be directed upwards 

 or downwards, and that in the case of so-called constant species 

 they do frequently equalize themselves ; but that more marked 

 variations, produced by more pronounced nutritive fluctuations, may 

 in a sense go on without limit, and then can only be restricted 

 and controlled by personal selection, that is, by the removal of the 

 ids concerned from the genealogical lineage of the species. 



In one direction variation can be proved to go on without limit, 

 and that is downwards, as is proved by the fact of the disappear- 

 ance of disused organs, for here we have a variation-direction, which 

 has been followed to its utmost limit, and which is completely inde- 

 pendent of personal selection ; it proceeds quite uninterfered tvith by 

 personal selection, and is left entirely to itself. It is a significant fact 

 that the disappearance of the individual parts of a larger organ, ac- 

 cording to all the data that are as yet available, proceeds at a very 

 unequal rate, so that it evidently depends to a great extent on chance 

 whether a disused part begins to degenerate sooner or later. Thus in 

 one of the Crustaceans livins: in the darkness of the caves of North 

 America the optic lobes and optic nerves have disappeared, while the 

 retina of the eye, the lens, and the pigment have been retained, and 

 in others the reverse has taken place, and the nerve-centres have per- 

 sisted while the parts of the eye have been lost (Packard). Variations 



II. K 



