196 A Defence of Mendel's 



Of hypotheses (&) and (c) the results of recrossing with 

 the two pure forms dispose ; and we can suggest no 

 hypothesis but (a) which gives an acceptable account of the 

 facts. 



It is the purity of the " extracted " recessives and the 

 " extracted ' : dominants primarily the former, as being 

 easier to recognize that constitutes the real proof of the 

 validity of Mendel's principle. 



Using this principle we reach immediately results of 

 the most far-reaching character. These theoretical de- 

 ductions cannot be further treated here but of the 

 practical use of the principle a word may be said. Where- 

 ever there is marked dominance of one character the 

 breeder can at once get an indication of the amount of 

 trouble he will have in getting his cross-bred true to either 

 dominant or recessive character. He can only thus fore- 

 cast the future of the race in regard to each such pair of 

 characters taken severally, but this is an immeasurable 

 advance on anything we knew before. More than this, it 

 is certain that in some cases he will be able to detect the 

 "mule" or heterozygous forms by the statistical frequency 

 of their occurrence or by their structure, especially when 

 dominance is absent, and sometimes even in cases where 

 there is distinct dominance. With peas, the practical 

 seedsman cares, as it happens, little or nothing for those 

 simple characters of seed-structure, &c. that Mendel dealt 

 with. He is concerned with size, fertility, flavour, and 

 numerous similar characters. It is to these that Laxton 

 (invoked by Professor Weldon) primarily refers, when he 

 speaks of the elaborate selections which are needed to fix 

 his novelties. 



We may now point tentatively to the way in which 

 some even of these complex cases may be elucidated by an 



