29 
Ridley .— The Distribution of Plants. 
There is no evidence to show that all these various widely dispersed 
species are of specially great antiquity, or that the area occupied by them 
depends primarily on their age, as suggested by Dr. Willis. It is certainly 
not the case in the plants which I have called weeds, where we often know 
the date of the introduction of the plant'into a given area. Their rapidity 
of dispersal depends on the area of suitable conditions for growth and their 
means of dispersal. On the other hand, we know that the Cycadeae are 
a very old group of plants now reduced to about eighty species, all 
remarkably local and confined to very limited areas, not one of which can 
be compared in extensive area-dispersal to the Reed, Ipomoea biloba , or 
Paspalum conjugatum, nor even to the Sanicle; while the Nipa palm 
inhabiting England in the Eocene period (for Nipadites Burtini is really 
hardly distinguishable from the Nipa of the present day) is now confined to 
the Indian Ocean from Ceylon and Bengal, down the Malay Peninsula 
to North Australia, and the Caroline and Solomon Islands, not having got so 
far as the Mascarene Islands, peninsular India, or Polynesia, in spite of its 
abundance as a drift plant both by seed and the large clumps of rhizome 
always to be seen drifting in Malayan seas. It is true it requires tidal mud 
for its growth, but there must be tidal-mud rivers in Samoa, Africa, and 
America quite suitable for this plant; we find, however, that, old as it is, it 
has a distribution now no wider than many of the doubtless more modern 
species which frequent the same area. 
An examination of what is known of the early floras of the Eocene and 
Miocene periods serves to show how local now are many of the genera 
existing in those times ; such instances are the genera Sequoia , Thujopsis , 
Salisburia , Tax odium, Andromeda , Cinnamomum (absent from America and 
Africa), Liquidambar , Plat anus, Hakea , and many others. 
Some of the older genera persist widely dispersed, as one might expect, 
occurring in both hemispheres, but many more have gradually disappeared 
and only persist now in a few isolated spots. 
It would be quite natural to imagine that plants of great age would be 
more widely dispersed throughout the world than more modernly evolved 
species, as having had more time for their dispersal, but the changes which 
the world has undergone since their evolution have been accompanied by 
extensive changes in the flora, the old species disappearing or persisting as 
endemics or very local plants in different corners, where they have held their 
own in spite of the fluctuations of climate and changes in the earth’s surface. 
The ecology, and especially the habitats and methods of dispersal, of each 
plant must be studied in the field before we can formulate any idea as to its 
history or the study of plant distribution in general. 
