INHERITANCE IN PURE LINES 47 



duced in the offspring. The tendency in these was to 

 reproduce the average character of the pure hne to 

 which they and their parent ahke belonged. This 

 general conclusion has been confirmed by various other 

 investigators with regard to several different kinds of 

 organisms. It seems to ai)ply generally, wherever 

 descent is habitually from one parent only. 



As to the causes of variation ^^^thin the pure line, 

 it seems probable that this is due entirely to environ- 

 mental conditions. The position of the bean on the 

 plant, the time at which it is formed, and the conditions 

 surrounding the mother plant itself are probably some 

 of these causes. In other words, it appears hkely that 

 differences within a pure line are all of the nature of 

 acquired characters, and we have seen that these are 

 probably not inherited. 



" A pure hne," says de Vries, " is completely constant 

 and extremely variable." This apparent paradox means 

 that variation occurs within a pure hne, as everywhere 

 in nature, yet the germ plasm, and therefore the heritage 

 of future generations, remains unaltered. 



It is evident, however, that heritable changes within 

 a pure line must sometimes arise, other-oise we could 

 conceive of no method by "uhich the different types 

 could have arisen. But it appears probable that these 

 changes are very rare, and that, when they do occur, 

 they are of the nature of sudden steps from one con- 

 dition to another — are mutations, in fact. 



The pure hne conception has brought about a very 

 considerable change in our notions of heredity. It was 

 formerly beheved that variation and change were the 

 universal rule amongst Hving things, and that perfect 

 constancy existed nowhere in nature. Whereas, within 

 the pure hne, a perfect constancy, very real although 

 somewhat obscured, exists, and real variations occur 

 only as rare exceptions. According to the old ideas, the 

 characters of a race were continually in a state of flux, 

 and selection in any particular direction could produce 

 change in that direction almost indefinitely. But it will 

 be seen, for example, that if we were to select, in a 



