Theories of Evolution 23 



inquiry into the real nature of the change. For 

 experimental purposes a single mutation does 

 not suffice; it must be studied repeatedly, and 

 be produced more or less arbitrarily, according 

 to the nature of the problems to be solved. And 

 in order to do this, it is evidently not enough 

 to have in hand the mutated individual, but it is 

 indispensable to have also the mutable parents, 

 or the mutable strain from which it sprang. 



All conditions previous to the mutation are to 

 be considered as of far higher importance than 

 all those subsequent to it. 



Now mutations come unexpectedly, and if the 

 ancestry of an accidental mutation, must be 

 known, it is of course necessary to keep ac- 

 counts of all the strains cultivated. It is evi- 

 dent that the required knowledge concerning the 

 ancestry of an accidental mutation, must neces- 

 sarily nearly all be acquired from the plants in 

 the experimental garden. 



Obviously this rule is as simple in theory, as 

 it is difficult to carry out in practice. First of 

 all comes the book-keeping. The parents, 

 grandparents and previous ancestors must be 

 known individually. Accounts of them must be 

 kept under two headings. A full description of 

 their individual character and peculiarities 

 must always be available on the one hand, and 

 on the other all facts concerning their heredi- 



