RESULTS OF PAL.EONTOLOGICAL RESEARCH 71 



divisions (Fig. 23). In the said succession the Sigillaria 

 form actual guides l for the various successive geological 

 horizons. In this way they divide themselves and 

 form as a whole a connected series by descent. The 

 actual causes and the purpose of the described trans- 

 formation must be deduced from observations of the 

 plants at present living. 



The G-inkgos show, particularly in the Jura forma- 

 tion, a great multiformity in the make of the leaves 

 which are of value as indicative characters for the 

 various systematic species and genera (specialization). 

 In a general way separate links may be determined 

 in the transformation of many other families which 

 eventually have led up to the present ones, but regard- 

 ing the details of the process we are not sufficiently 

 instructed. 3 



(2) Phenomena of Convergence (p. 45). 



By convergence is understood the formation of like 

 or very similarly constituted organs, or, if the organs 

 already exist, of similarities of structure and form in 

 organisms which according to their total type belong to 

 different systematic categories. Under some circum- 

 stances the limitations of such converging groups may 

 naturally be rendered difficult. 



It has long been remarked that, for instance, 

 the Carboniferous flora appears outwardly of a fairly 



1 Gothan : Entwicklung tier Pflanzenwett, p. 41. 



2 Reinke : Philosophic dcr Botanic, p. 136. ' Actual transitional forms 

 between Tertiary species and living species cannot be followed up with the 

 desired clearness.' (Reinke speaks here of flowering plants.) 



