THEORY OF DESCENT. 845 



represent the ancestors of the Vascular Cryptogams, and from this branch of the 

 tree the Ferns, Equisetaceae, Ophioglossaceae, Rhizocarpese, and Lycopodiaceae, 

 would proceed as branches which themselves further ramify. Where the branch is 

 given off for the heterosporous Vascular Cryptogams would be situated the primitive 

 forms of Phanerogams, beginning with the Cycadeas, and producing by further 

 ramifications the Coniferse, Monocotyledons, and Dicotyledons \ There is still much 

 uncertainty in this plan, but the greater the progress made by a severe method of 

 investigation and with the light of the theory of descent, the more nearly will 

 it be possible to build up the family-tree and to give it a distinct form. 



The theory of descent requires that the various forms of plants must have arisen 

 at different times, that the primitive forms of the separate classes and groups existed 

 at an earlier period than the derived ones ; and palaeontological research, although at 

 present it has but a very small amount of material at its disposal, supports this view. 



In the same manner it is a necessary consequence of the theory that each plant- 

 form must have originated at a definite spot, that it must have spread gradually more 

 widely from that spot, that its change of locality in the course of generations must 

 have depended on climatic conditions, the competition of rivals, &c., and that its 

 distribution must have been impeded by hindrances or assisted by means of 

 transport^. The geographical distribution of plants has already determined in 

 the case of many forms the spots on the surface of the earth or centres of distri- 

 bution from which they gradually spread ; it has shown how the distribution has been 

 hindered sometimes by climate, sometimes by chains of mountains, sometimes by 

 seas ; how more recently formed islands have been peopled by the plants from the 

 neighbouring continents which have become the ancestors of new species^; how 

 some species when transported to a new soil (as European plants in America and 

 vice versa) have sometimes carried on a successful struggle for existence with the 

 native plants and have increased enormously. In the distribution of plants at present 

 existing, as for instance Alpine plants, it is possible to recognise the influences of 

 the last great geological changes, of the entrance and disappearance of the glacial 

 epoch and of earlier periods. 



' [In the fourth edition of his ' Lehrbuch,' recently published, Sachs has united Algre and Fungi 

 into one group (see Appendix, p. 847). He has also withdrawn the pedigree of the vegetable king- 

 dom sketched in the text, and has substituted for it (p. 9 1 8) the following remarks : — 



' Frequent attempts have been made to draw up such a so-called " genealogical tree" either for 

 the whole or some part of the vegetable kingdom. Up to the present time these attempts have not 

 proved very satisfactory. Our knowledge of the true relationships is still very imperfect ; too much 

 room is consequently left for fanciful speculation and the influence of subjective impressions. 

 I shall content myself therefore with pointing out that in drawing out such a genealogical tree the 

 closest attention must be paid to the simplest existing forms of the different types or classes ; the 

 relationship to the common primitive parent-forms will reveal itself most distinctly in these. From 

 each of these simplest forms, however slightly different, a ramifying series may be derived ; variation, 

 proceeding independently in each series, will separate the series themselves still further ; and the most 

 perfect forms of the different types will therefore differ the most widely from one another.' — Ed.] 



2 Kerner has given an illustration of what can be accomplished in this direction in the rela- 

 tionships, geographical distribution, and history of the species of Cytisus from the primitive form 

 Tubocytisus, in his pamphlet Die Abhangigkeit der Pflanzengestalt von Klima und Boden ; Inns- 

 bnick, 1869. 



3 See Dr. Hooker, On Insular Floras, Gardener's Chronicle, Jan. 1867 ; Ann. des sci. nat. 

 5th series, vol. IV, p. 266. 



