The Boundaries Betzveen Related Species. Ml 



of the current theory of selection. It makes it possible 

 to pick out series of individuals which belong to related 

 but different species and (being careful to choose suit- 

 able characters) to arrange them in such a way that they 

 form a perfectly continuous series from one end to the 

 other. If no gaps have been left in such groups by the 

 death of species or if there are sufficient species left, 

 perfectly continuous series of this kind can be arranged 

 of almost any desired length. 



But, as a rule, this can only be done by dealing with 

 single characters and by not being particular about the 

 number of individuals which go to form the successive 

 steps in the series. 



An example will make my meaning clear. Oenothera 

 Lamarckiana differs from 0. biennis by the beauty and 

 size of its flowers. The two species can be distinguished 

 at a great distance. The petals of the former are twice 

 as long as those of the latter. But in both species the 

 leno^th is variable and follows Ouetelet's law of indi- 

 vidual variability, being in a high degree dependent on 

 nutrition. Petal-length also exhibits partial variability 

 and is especially low at the end of the flowering period 

 when the plant is exhausted by bearing seed. The small- 

 est flowers are found on the main stems of plants which 

 are nearly over, on small lateral branches or on indi- 

 vidually weak plants : the largest on well nourished plants 

 just beginning to flower, or on vigorous lateral branches 

 of large plants which through some accident or other 

 have lost their main stem. This is true both of plants in 

 the field and of those in the garden. 



If now we choose the largest flowers of 0. biennis 

 and the smallest of 0. Lamarckiana, we shall find tiiat 



