194 Prof. Asa Gray on the Question 
should wear out or deteriorate from any inherent cause. The 
transient existence or the deterioration and disappearance 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 difficult 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Or they go out of fashion and are superseded by others thought 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. Asoffspring 
tend to resemble grandparents almost as much as parents, and 
as a line of close-bred ancestry is generally prepotent, so newly 
originated varieties have always a tendency to reversion. This 
is pretty sure to show itself in some of the progeny 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 im 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, Wil 
varieties propagated from buds (7. e. by 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 afford the slightest pros- 
