176 ESSAYS. 
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and are superseded by others thought to be better, and so the 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 revert to an ancestral form. As offspring 
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 rever- 
sion. ‘This is pretty sure to show itself in some of the pro- 
geny 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 immediate parent, 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 unlim- 
ited 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 from buds, % 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 
testimony has all along been conflicting. Andrew Knight 
was sure that they do, and there could hardly be a more 
trustworthy witness. 
“The fact,” he says, fifty years ago, “that certain varieties 
of some species of fruit which have been long cultivated can- 
not now be made to grow in the same soils and under the 
