Primitive Angiosperms. 175 
and it has proved barren. No consistent explanation of the derivation 
of Dicotyledons from a monocotylar race has yet been offered which would 
account for the difference in stem anatomy and in some minor features which 
exists between the two classes. This is in itself a strong argument against 
the truth of the monocotylar view, and serves to confirm the evidence in 
favour of crediting the Primitive Angiosperms with two cotyledons. 
Assuming that the seedling of Primitive Angiosperms was dicotylar, 
we have to account for the origin of Monocotyledons. Most of the few 
botanists who upheld this view in the last century supposed the single 
cotyledon found in this class to represent one of the ancestral pair, the 
second having been suppressed. Agardh, however, suggested in 1829 that 
in some Monocotyledonous families the single cotyledon might represent 
a fusion of the Dicotyledonous pair. This suggestion seems to have been 
founded partly on a misinterpretation of the seed-structure of Nymphaea 
and its allies, and it fell into oblivion. 
The most important paper published of late years in support of the 
first of these alternatives is that of Professor Henslow (published in the 
Linnean Journal for 1892 ( 38 )). He assumes a common origin for both 
classes, and collects a quantity of evidence to show : — 
1. That Monocotyledons are derived from a stock essentially Dicotyle- 
donous ; that is, possessing two cotyledons, a true cambium, and some 
of the minor characters of Dicotyledons. 
2. That the single cotyledon of Monocotyledons represents one of those 
possessed by the ancestral stock, the other having been suppressed. 
3. That the features characteristic of Monocotyledons were evolved 
during the adaptation of one or more branches of the ancestral stock to an 
aquatic habit. 
With regard to the first proposition we have seen that existing evidence 
is altogether in favour of supposing Primitive Angiosperms to have possessed 
a true cambium and two cotyledons. The evidence advanced in support 
of the two succeeding propositions may be criticized. 
Professor Henslow points out that a very large proportion of Mono- 
cotyledonous orders are aquatic — about 33 per cent, as compared with 
4 per cent, of Dicotyledonous orders. When a class or an order contains 
aquatic forms only, it is reasonable to conclude, in the absence of evidence 
to the contrary, that the common ancestor of that class or order was aquatic 
also. But the existence of a large minority of aquatic orders within a class 
may be equally well explained on grounds other than the descent of all 
members of that class from an aquatic stock. In this case I believe that 
Monocotyledons may be shown to be on the whole a decadent race of 
which some branches have been driven to an aquatic habit to escape the 
severer competition on land. 
