24 Descent 



tary qualities. These are to be deduced from 

 the composition of the progeny, and in order 

 to obtain complete evidence on this point, two 

 successive generations are often required. The 

 investigation must ascertain the average condi- 

 tion of this offspring and the occurrence of any 

 deviating specimens, and for both purposes it 

 is necessary to cultivate them in relatively large 

 numbers. It is obvious that, properly speak- 

 ing, the whole family of a mutated individual, 

 including all its nearer and more remote rela- 

 tives, should be known and recorded. 



Hence pedigree-book-keeping must become 

 the general rule. Subordinate to this are two 

 further points, which should likewise be stated 

 here. One pertains to the pure or hybrid 

 nature of the original strain, and the other to 

 the life-conditions and all other external in- 

 fluences. It is manifest that a complete under- 

 standing of a mutation depends upon full infor- 

 mation upon these points. 



All experiments must have a beginning. The 

 starting-point may be a single individual, or a 

 small group of plants, or a lot of seeds. In 

 many cases the whole previous history is ob- 

 scure, but sometimes a little historical evidence 

 is at hand. Often it is evident that the initial 

 material belongs to a pure species, but with re- 

 spect to the question of elementary species it is 



