Mutability 75 
In contrast to these changes of the internal causes, the ordinary 
variability which is exhibited during the life-time of a species is 
called fluctuating variability. The name mutations or mutating 
variability is then given to the changes in the specific characters. 
It is desirable to consider these two main divisions of variability 
separately. 
In the case of fluctuations the internal causes, as well as the 
external ones, are often apparent. The specific characters may be 
designated as the mean about which the observed forms vary. Almost 
every character may be developed to a greater or a less degree, but 
the variations of the single characters producing a small deviation 
from the mean are usually the commonest. The limits of these fluctua- 
tions may be called wide or narrow, according to the way we look at 
them, but in numerous cases the extreme on the favoured side 
hardly surpasses double the value of that on the other side. The 
degree of this development, for every individual and for every organ, 
is dependent mainly on nutrition. Better nourishment or an increased 
supply of food produces a higher development; only it is not always 
easy to determine which direction is the fuller and which is the poorer 
one. The differences among individuals grown from different seeds are 
described as examples of individual variability, but those which may 
be observed on the same plant, or on cuttings, bulbs or roots derived 
from one individual are referred to as cases of partial variability. 
Partial variability, therefore, determines the differences among the 
flowers, fruits, leaves or branches of one individual: in the main, it 
follows the same laws as individual variability, but the position of a 
branch on a plant also determines its strength, and the part it may 
take in the nourishment of the whole. Composite flowers and umbels 
therefore have, as a rule, fewer rays on weak branches than on the 
strong main ones. The number of carpels in the fruits of poppies 
becomes very small on the weak lateral branches, which are pro- 
duced towards the autumn, as well as on crowded, and therefore on 
weakened individuals. Double flowers follow the same rule, and 
numerous other instances could easily be adduced. 
Mutating variability occurs along three main lines. Hither a 
character may disappear, or, as we now say, become latent; or a 
latent character may reappear, reproducing thereby a character 
which was once prominent in more or less remote ancestors. The 
third and most interesting case is that of the production of quite 
new characters which never existed in the ancestors. Upon this 
progressive mutability the main development of the animal and 
vegetable kingdom evidently depends. In contrast to this, the two 
other cases are called retrogressive and degressive mutability. In 
nature retrogressive mutability plays a large part; in agriculture 
