Systematic Atavism 633 



allied species. It brings before us, and empha- 

 sizes the importance of the conception of the 

 so-called unit-characters. 



The primrose will serve as an example. In 

 the second lecture we have seen that the old 

 species of Linnaeus, the Primula vera, was 

 split up by Jacquin into three smaller ones, 

 which are called P. oficinaUs, P. elatior and P. 

 acaiilis. From this systematic treatment we 

 can infer that these three forms are assumed to 

 be derived from a common ancestor. Now two 

 of them bear their flowers in bracted whorls, 

 condensed into umbels at the summits of a scape. 

 The scapes themselves are inserted in the axils 

 of the basal leaves, and produce the flowers 

 above them. In the third species, Primula 

 acaulis, this scape is lacking and the flowers are 

 inserted singly in the axils on long slender 

 stalks. For this reason the species is called 

 acaulescent, indicating that it has no other 

 stem than the subterranean rootstock. But on 

 closer inspection we observe that the flower- 

 stalks are combined into little groups, each 

 group occupying the axils of one of the basal 

 leaves. This fact at once points to an analogy 

 with the umbellate allies, and induces us to ex- 

 amine the insertion of the flowers more crit- 

 ically. In doing so we find that they are united 

 at their base so as to constitute a sessile umbel. 



