258 A TEXTBOOK OF GENERAL BOTANY 
In natural selection it is those plants that are not fitted to 
their environment, or for withstanding competition, that are 
eliminated, while in artificial selection the plants that are dis- 
carded are those thought to be least serviceable to mankind. 
It frequently happens, therefore, that mutations which would be 
selected and thus preserved by man are very different from those 
which are preserved by natural selection. Such a mutation as 
a seedless orange is desirable from the standpoint of man but 
could not continue to exist in nature. From the standpoint of 
a plant subject to natural selection many seeds and a compara- 
tively small amount of pulp are favorable characters, 
In nature variegated leaves with white areas resulting from 
a lack of chlorophyll would be a disadvantage, as the material 
used in constructing these areas would be wasted as far as the 
chief function of the leaves is concerned. From the standpoint 
of artificial selection such plants are frequently desirable on 
account of their ornamental value. In nature such variegated 
leaves are very rare, but they are very common in cultivation. 
Under natural conditions any mutation the result of which 
is unfavorable to the preservation of the species will be elimi- 
nated by natural selection, but there is a tendency for favorable 
mutations to be preserved. 7 
From the foregoing discussion it is easy to understand why 
many of our cultivated plants cannot survive when left to them- 
selves. Also, it is evident why plants seem thoroughly adapted 
to their environment. 
Significance of sexuality. The value of sexuality is a subject 
about which there has been much dispute. All species of higher 
animals are composed of males and females. This results in the 
production of only half as many offspring as there would be if 
the animals were bisexual or reproduced by a vegetative proc- 
ess. The great majority of seed plants also reproduce sexually. 
Moreover, most of them are fitted with some device to insure 
cross-pollination. Considering that plants are subject to a rigorous 
natural selection, and that still the majority reproduce sexually, 
it would seem that this method must afford a real advantage. 
