through Self-adaptation to a Moist or A quatic Habit. 719 
comparison with what must have been the whole land surface of the globe 
during that prolonged period. 
That Monocotyledons should have arisen nearly contemporaneously 
with Dicotyledons is not surprising if my contention be true ; because 
rivers and lakes as well as morasses, we may be sure, abounded in all ages. 
It has been suggested that the most primitive plants were aquatic, and 
that some took to land and became terrestrial. Unfortunately, Geology 
has revealed nothing to support this theory or the reverse ; but that all 
Dicotyledons now existing have been evolved from xerophytic Gymno- 
sperms is generally admitted, and that Monocotyledons, as now limited, 
were derived from the former is becoming generally accepted. There are 
about a dozen genera of the order Naiadaceae, all of which are aquatic 
plants, most of them living in the sea ; but that they are degradations and 
not primitive is obvious. No existing order of either class can be said to 
be primitive, because of their simple structure ; for this, in all cases, is 
the result of degradations. The same must be said of fresh-water plants. 
3. Distribution and Percentages of the Natural Orders of 
Monocotyledons as compared with those of Dicotyledons. 1 
If it be asked where Monocotyledons were evolved, the present dis¬ 
tribution of this class may perhaps indicate the region. Of the thirty-two 
orders described by Bentham and Hooker in the ‘ Genera Plantarum \ 
twenty-one are chiefly found in the Tropics and warmer regions of the 
Temperate Zones; whereas only six have species reaching the Arctic and 
Antarctic regions. 
Sir J. D. Hooker, in his Introductory Essay to the ‘ Flora of Tasmania ’, 
tells us that there are the following proportions between Monocotyledons 
and Dicotyledons :— 
Europe. 
Russian Empire 
British N. America 
South Africa 
Australia 
Temperate Floras. 
Monocotyledons : Dicotyledons 1 : 5*2 
1 : 5-i 
Tropical Floras . 
W. Tropical Africa ..... 
Ceylon ..... 
India ..... 
Tropics generally ..... 
Australia ..... 
This table, therefore, illustrates the same phenomenon. 
1 loc. cit., p. 487. 
3 E 2 
