194 Prof. Asa Gray on the Question 



should wear out or deteriorate from any inherent cause. Tiie 

 transient existenee or the deterioration and disap})earance of 

 many such races is sufficiently accounted for otherwise — as, in 

 the case of extraordinarily exuberant varieties, such as mam- 

 moth fruits or roots, by increased liability to disease, already 

 adverted to, or by the failure of the high feeding they demand. 

 A common cause, in ordinary cases, is cross-breeding, through 

 the agency of wind or insects, which is dilHcult to guard against. 

 Or they go out of fashion and are superseded by others thought 

 to be better ; and so the old ones disappear. 



Or, finally, they may revert to an ancestral form. Asoflfspring 

 tend to resemble grandparents almost as much as ])arents, and 

 as a line of close-bred ancestry is generally ])rcpotent, so newly 

 originated varieties have always a tendency to reversion. This 

 is j^retty sure to show itself in some of the ])rogeny of the 

 earlier generations ; and the breeder has to guard against it by 

 rigid selection. But the older the variety is (that is, the longer 

 the series of generations in which it has come true from seed), 

 the less the chance of reversion : for, now, to be like the imme- 

 diate parents is also to be like a long line of ancestry ; and so all 

 the influences concerned (that is, both parental and ancestral 

 heritability) act in one and the same direction. So, since the 

 older a race is the more reason it has to continue true, the 

 presumption of the unlimited permanence of old races is very 

 strong. 



Of course the race itself may give off new varieties ; but 

 that is no interference with the vitality of the original stock. 

 If some of the new varieties supplant the old, that will not be 

 because the unvaried stock is worn out or decrepit with age, 

 but because in wild nature the newer forms are better adapted 

 to the surroundings, or, under man's care, better adapted to his 

 wants or fancies. 



The second question, and one upon which the discussion 

 about the wearing-out of varieties generally turns, is, Will 

 varieties propagated Jy-om buds {i. e. hy division), grafts , bulbs , 

 tubers, and the like necessarily deteriorate and die out ? First, 

 Do they die out as a matter of fact? Upon this the testi- 

 mony has all along been conflicting. Andrew Knight was 

 sure that they do ; and there could hardly be a more trust- 

 worthy witness. 



'^ The fact," he says, fifty years ago, " that certain 

 varieties of some species of fruit which have been long culti- 

 vated cannot now be made to grow in the same soils, and under 

 the same mode of management which was a century ago so 

 perfectly successful, is placed beyond the reach of controversy. 

 Every experiment which seemed to aff'ord the slightest pros- 



