THEORY OF DESCENT. 
943 
classes and groups described at length in Book II might be represented by lines, 
which should express their actual affinity to one another ; and the system of 
diverging lines which would thus be obtained might be compared to an irregular 
system of branching. 
It has frequently been attempted to draw up genealogical trees for the whole 
Vegetable Kingdom and for the various groups of it, but the attempts made 
hitherto are by no means satisfactory, for the incompleteness of our knowledge 
as to the real relationships leaves too much room for the play of the imagination 
and of subjective impressions. I will merely suggest here, as I did repeatedly in 
Book II, that in endeavouring to form such a genealogical tree, the simplest forms 
of the various types or classes must be especially considered, for it is in them 
that the evidences of descent from common ancestral forms will most easily be 
detected. With each of these simple forms, which do not differ widely from each 
other, is connected a branching developmental series in which variation is taking 
place independently of other series ; in consequence of this variation the difference 
between the members of one series and those of another is increasing, so that the 
most perfect forms of different types are those which are the most widely separated. 
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, (fee, 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 have gradually spread ; it has shown how the distribution 
has been hindered sometimes by climate, sometimes by chains of mountains, some- 
times 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. 
When we reflect what a number of generations our cultivated plants must have 
passed through before any considerable amount of new properties were manifested in 
^ 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 Abhängigkeit der Pflanzengestalt von Klima und Boden, Inns- 
bruck 1869. 
^ See Dr. Hooker, On Insular Floras, Gardener's Chronicle, Jan. 1867; Ann. des sei. nat, 
5th series, vol. IV. p. 266. [Wallace, Island Life, 1880,] 
