516 Influence of Nutrition and Selection. 



fied with being a purely descriptive and statistical one; 

 it must, like every other, seek to determine the causes of 

 the phenomena of which it treats. 



If polymorphism is excluded on the one side and 

 mutability on the other, the whole range of variability 

 can be described in terms of Quetelet's law. Then 

 there is the question of the inheritance of these varia- 

 tions. The deviations of the various individuals from 

 the mean are heritable : but not in their entirety ; a part 

 is always lost. Regression always takes place, and this 

 usually involves more than one-half and often as much 

 as two-thirds of the original deviation. This is the 

 source of the third principle in the theory of variability : 

 the possibility of an increase of the deviation by means 

 of selection. This increase, which is sometimes spoken 

 of as a heaping up of similar small differences, leads to 

 the so-called accumulation and fixation of characters and 

 thus to the production of improved races. 



Exactly the same deviations from the mean as those 

 with which statistics have made us familiar may be 

 brought about, either by chance or by deliberate experi- 

 ment, by changes in the conditions of nutriment. Char- 

 acters and organs whose dimensions may be increased or 

 diminished by selections, are also dependent on the con- 

 ditions of life and in many cases it is very difficult to 

 decide which of the two causes has been most operative. 



The recent researches of Mac Leod and others clearly 

 point to a very close relationship between nutrition and 

 variability. For, broadly speaking, variability is really 

 nothing more than differences in individual strength. 

 The stronger a plant or a branch on a plant is, the greater 

 is the likelihood of deviations in a positive direction ; weak 



