256 A TEXTBOOK OF GENERAL BOTANY 
is frequently utilized in breeding. A conspicuous example is 
that of the mule, obtained by crossing a mare and a male ass. 
In plants many valuable hybrids are reproduced vegetatively 
by buds or by cuttings. This method of preserving a hybrid is 
very frequently practiced with fruit trees and ornamental plants. 
While crossing different strains frequently results in increased 
vigor, it does not follow that a good variety will be improved by 
crossing with a poor one; in fact, the reverse is usually the case. 
It is, of course, not to be expected that a valuable variety can be 
improved by the incorporation of undesirable characters in it. 
Combination of outbreeding and inbreeding. When a plant can 
be reproduced readily by vegetative means, it is a simple matter 
to retain the desirable characters found in a hybrid. The case 
is very different, however, with organisms that are reproduced 
only by the sexual process. In such eases, if a hybrid is hetero- 
zygous for many desirable characters, a strain that will breed 
true for a considerable proportion of them can frequently be 
obtained by inbreeding. Very valuable results may be produced 
by such inbreeding accompanied by vigorous selection. 
NATURAL SELECTION 
‘The method of improving cultivated plants has been that of 
selection, which may or may not have been accompanied by 
hybridization. Not only is selection the method of improve 
ment but, moreover, continued selection is frequently necessary 
if the best qualities of cultivated plants are to be maintained. 
Naturally all injurious mutations should be eliminated, as should 
all undesirable individuals resulting from hybridization. 
Since selection plays such a prominent part in the growing 
of cultivated plants and domestic animals, it is not surprising 
that in nature too plants and animals are subject to a process 
of selection. This selection by nature is called natural selection, 
to distinguish it from artificial selection, or selection by man. 
Struggle for existence. Observation of natural conditions indi- 
cates very clearly that, except where man has interfered, a given 
