2i6 BREEDING 



cerned every time fertilisation takes place is certainly 

 very large ; it is nothing less than the sum total of 

 the characters of the organism in question. Accord- 

 ing to this generalised Mendelian theory, the organ- 

 ism is made up of a number of characters which are 

 called unit-characters, because they are transmitted 

 as independent units in inheritance. These unit- 

 characters were, in the early days of Mendelian 

 speculation, considered to be associated in pairs, but 

 as explained in Chapter IX., the pair is now regarded 

 as consisting in the presence of a particular character 

 as its dominant member, and the absence of this 

 character as its recessive member. But this is a 

 secondary feature of the theory. The essence of it 

 is that the organism is built up of an obviously 

 immense number of separately transmissible unit- 

 characters, the number, limits, and nature of which 

 can be determined by experimental breeding. With 

 regard to the soundness of this theory, all we know 

 at present is that it applies to the relatively small 

 number of characters which have been dealt with in 

 Mendelian studies. This knowledge is sufficient to 

 justify its application to practical problems, if there 

 is reason to believe that the inheritance of the here- 

 ditary characters under consideration is of, or approxi- 

 mates to, the Mendelian type. But this knowledge 

 is not as yet by any means sufficient to warrant even 

 the hope that the future problems of heredity will 

 be solved by its aid. I am, however, merely con- 

 cerned here in enunciating this theory, and not in 

 estimating its truth. 



