428 The Systematic Value of the New Species. 



this character of petal length overlaps in the two cases. ^ 

 For in these extreme cases the flowers of biennis are 

 actually larger than those of Lamarckiana.^ 



If we have collected a number of flowers including 

 such extremes, it is obviously an easy matter to arrange 

 an unbroken series with the smallest of biennis at one 

 end and the largest of Lamarckiana at the other. The 

 limits between the two species cannot be detected in such 

 series even by the practised eye. And yet 0. bie finis and 

 O. Lamarckiana never merge into one another. 



If we wish to extend the series we can do so by adding 

 the small-flowered 0. miiricata to it in exactly the same 

 way.*^ And, if we leave relationship out of account, we 

 could extend continuously down to Oenothera mimiti- 

 flora with its flower no longer than a millimeter. Such 

 series can be arranged for almost any character in the 

 vegetable kingdom and in infinite variety.^ They con- 

 fuse the limits drawn between related species as far as 

 the several characters are concerned. 



If, in the classification of animals and plants, we fix 

 our attention on a single character we shall always en- 

 counter long unbroken series of this kind. The shells 

 of snails and the wings of butterflies are examples. It 



* That this must generally be the case may be derived from the 

 law of variability. Imagine two curves of variation drawn on the 

 same abscissa. The greater the number of individuals included the 

 further will the limits of the curves extend until they, first, touch and 

 ultimately overlap. It is further obvious that the likelihood of this 

 happening, even with a small number of individuals depends directly 

 on the closeness of the tops of the curves (the mean values of the 

 characters) and on the amplitude of the curve, or the degree of var- 

 iabihty (Q). 



^ Examples in the following section. 



^ See the following section. 



* For example the narrowest leaves of Typha latifolia are nar- 

 rower than the broadest leaves of T. angustifolia. 



