138 BREEDING 



five heavily pigmented, brown bands in the snail 

 of the hedgerows [Helix hortensis) is recessive to the 

 pale yellow, entirely bandless condition. Again, it 

 is not, at present, easy to see how the fact, discovered 

 by Mr. Biffen, that susceptibility to the attacks of 

 " rust " in wheat is dominant to immunity from such 

 attacks can be brought in line with this hypothesis. 

 In the case of the snails, it must be supposed that the 

 possession of bands is due to the absence of some- 

 thing which is present in the pale yellow snail 

 and prevents them appearing. In the case of the 

 wheat it must be supposed that resistance is due to 

 the presence of some anti-toxin, and susceptibility 

 to the presence of some substance which prevents 

 the formation of the anti-toxin. With further 

 inquiry the scope of this hypothesis can be better 

 estimated ; at present its chief service is that it 

 stimulates this inquiry. 



Another point raised by this Presence and 

 Absence hypothesis can be brought out by comparing 

 it with Prof, de Vries's conception of the Mendelian 

 pair. This theory, as stated in the first chapter, 

 is that specific characters blend on crossing ; but 

 that varietal ones exhibit Mendelian phenomena. 

 The reason for this is that the specific characters are 

 new creations, isolated and without partners, whilst 

 varietal characters are the result of the transference 

 of an already existing character from one state to 

 another, as, for instance, from the patent to the 

 latent, or vice versa. Thus when one organism is 

 crossed with another from which it differs in the 



