544 TRANSMISSION 



EFFECT OF CONTINUED SELECTION UPON VARIABILITY AND TYPE l 





SECTION XVIII DURATION OF VARIETIES, BREEDS, 

 AND FAMILY STRAINS 



How long can a desired breed or family be retained? There is 

 a popular belief that varieties wear out, and that breeds must 

 of necessity be constantly reenforced by new material or by new 

 combinations to take the place of worn-out stock. 



The facts just presented, however, clearly indicate that if a 

 type does not remain true indefinitely either it is the fault of 

 adverse selection, accidental or otherwise, or else it is due to 

 some physical or biological cause, for the type, once obtained, 

 naturally breeds true. 



Again, from the fact that variability is not greatly reducible, 

 we are safe in assuming that types once established by selection 

 will not only remain true but are capable of still further develop- 

 ment if we bestow additional attention and selection, and that 

 the upper limits of improvement are fixed, if fixed at all, by 

 some circumstance other than variability. It may be biological, 

 like loss of fertility or reversal of selection, or it may be 

 mechanical, but the cause, whatever it may be, that sets a limit 

 to improvement is not connected with variability. 



In the last analysis, however, we are bound to raise the ques- 

 tion whether all types can be indefinitely maintained, even by the 



1 Pearson, Grammar of Science, p. 485. 



