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. Either 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 



