534 Engler . — Plants of the Northern Temperate Zone in their 
between the ordinary C. caespitosum and C. africanum , (Hook, f.) Oliv., 
having green sepals, larger petals, and acute leaves. 
All these species mentioned so far are closely allied to plants widely 
distributed in Europe or more generally throughout the northern 
temperate zone of the Old World, growing there in the lower as well 
as the upper regions, whereas, in Africa, they are to be found only in 
the upper or uppermost belts. For several of them, their seeds being 
small and light, it may be assumed that they have been distributed by 
heavy gales ; still stronger seems the argument that the first transport 
from Europe or Western Asia to the upper treeless regions of Africa 
has been effected by birds of passage, the seeds adhering to their feet 
with soil or by hairs and bristles to the plumage, and that they were 
dispersed afterwards from mountain to mountain by winds. It is not 
impossible that in this manner seeds are reaching Africa from the 
northern temperate zone even at the present time. But most of these 
species are not only to be found now on several of the high mountains 
of Africa, but they appear there in forms constantly distinct from the 
European ones. It seems reasonable, then, to assume that immigration 
took place at some earlier date. On the other hand, from our experiences 
of alpine plants cultivated in the lowlands and growing there more 
vigorously, I am led to the opinion that the forms modified by a longer 
period of vegetation and by higher temperature have arisen in a relatively 
shorter length of time. The principal reason for my supposing that 
immigration took place at some earlier date is the probable state of 
external conditions during that pluvial epoch as assumed by geologists, 
which must have been more favourable for any immigration of forms 
of the temperate zone than the conditions prevailing at the present time. 
Extensive alluvial deposits of a recent date, traces of a formerly larger 
amount of water in the present lakes and rivers, traces of a wider extension 
of African glaciers in the past (Hans Meyer on Kilimanjaro, in Zeitschr. 
Gesellsch. Erdkunde, Berlin, 1904, p. 193) serve as a proof of this 
pluvial epoch. During this period the forests must have extended further 
down, the treeless tracts spread to lower regions. At this time, then, 
the areas suitable for plants of more temperate zones were larger and 
approaching each other more, though not really continuous. There has 
always been also a large region between Abyssinia and Southern Europe 
or Western Asia which has never been suited to harbour the highland 
plants mentioned by me. 
All these highland forms have a common feature in being syste- 
matically isolated in Tropical Africa, whereas there is quite a number 
of allied species in the temperate zone. This is easily to be accounted 
for, because, in the present time as well as during the pluvial epoch, 
only the loftiest mountains of Tropical Africa afforded conditions which 
