n 
FLORA OF ADEN. 
overwhelming evidence taken from the Jurassic fossils as well as from, 
the Cretaceous deposits. The great revolutions in physical geography, 
which took place towards the end of the Cretaceous and during early 
Tertiary times, resulted in the break-up of the old continent, and were 
followed by the rise of the Himalayan range. 
At the same time, perhaps, or at a later period a great depression of 
land took place which resulted in the formation of the Red Sea and 
consequently in the separation of what is now called Arabia and Abyssi- 
nia. Along both shores of the Red Sea and of the Gulf of Aden there is 
known to be a great development of volcanic rocks. There is every proba- 
bility that these, so far as they are known, belong to one series, and as 
some active volcanoes still exist in the sea, and cones quite unchanged 
in form and evidently of recent date abound in many places along the 
coast, it is clear that the series is still in progress of formation, and 
that it is, in part at least, of recent date. Although there can be 
little doubt of the recent date of a large portion of this series, the great 
amount of denudation which parts of it have undergone around Zulla 
and Massowa and also at Aden seem to prove that these portions of the 
series are of a more ancient date, but not older than the Pliocene rocks. 
It is therefore impossible that the flora of Aden developed independ- 
ently during a geological period of long duration. We must, on the 
contrary, conclude, that it is the product of species imported from the 
neighbouring countries. This view is strengthened by the fact that the 
plants endemic in Aden are very few in number. 
9. Means of dissemination. 
If the flora of Aden, as we have shewn above, is the product of 
immigration, it is evident that the plants must be provided with special 
modifications facilitating migration from one place to another. 
Without entering into a detailed description of those modifications we 
wish to show the possibility of migration of the plants of Aden under 
the action of the chief distributing agents, viz. wind, water, animals, 
man, and mechanical propulsion. 
(a) Wind. 
Practically all terrestrial plants in which modifications for increasing 
the surface of seed or fruit have been greatly developed belong to the 
group of wind-distributed species. Winged, sack-like, hairy, plumed and 
