8o HEREDITY AND VARIATION 



The array of closely-packed rods will then form a plane area, 

 bounded by straight lines at its sides and along its base, but by 

 a flowing curve above, which takes note of every one of the 

 values on which it is founded, however immense their multitude 

 may be. The shape of the curve is characteristic of the 

 particular group of values to which it refers, but all arrays 

 have a family resemblance due to similarity of origin ; they 

 all drop steeply at one end, rise steeply at the other, and 

 have a sloping back. An array that has been drilled into some 

 such formation as this, is the tactical unit of the new statistics. 

 Its outline is expressed by a general formula whose constants 

 are adapted to each particular case, and being thus brought 

 within the grip of mathematics, the internal relations of an 

 array and their relations to those of any other array can 

 be expressed in exact numerical forms." {Biometrika, vol. i., 

 1901, p. 7.) 



Theory of Evolution by Selection of Fluctuating Varia- 

 tions. — It is certain that most offspring differ from their parents 

 in many quantitative details. It is certain that when measure- 

 ments are taken of a large number of individuals of the same 

 species in reference to a particular character, the results, when 

 plotted out, conform approximately to the normal curve of 

 frequency. If measurements be taken in a subsequent genera- 

 tion there is a similar result, but the curve need not be precisely 

 the same. The mode of the curve — i.e., the most frequently 

 occurring dimension of the measured character, may change 

 from one generation to another. It is usually beUeved that one 

 of the ways in which this change can be effected is by natural 

 selection. But to think of new species arising by slow changes 

 of this sort is in many ways difficult, apart altogether from the 

 fact that definite demonstration of the operation of selection 

 has been rarely attempted. 



(i) Such a character as a Roman nose is certainly heritable, 

 though it is not always inherited. But we cannot speak so 



