through Self-adaptation to a Moist or Aquatic Habit. 741 
In Ceratozamia it has been shown above that as a seed lies flat on 
moist ground, the cotyledon on the lower side only is developed ; but if 
the seed be kept uniformly moist all round, then both of the cotyledons 
become developed. 
Now, seeds (provided with a fitting temperature) will not germinate 
unless water be present, either within the seed or acquired from without; 
so that whatever may be the differential causes acting on the two cells or 
groups of cells of the pro-embryo, whence originate the two cotyledons, if 
one receive a trifle more moisture than the other, it may, perhaps, determine 
the development of the former in xerophytes, and the latter in hydrophytes. 
Miss Sargant has also shown that the two cotyledonary strands, corre¬ 
sponding to the two cotyledons, one of which is present, but carries the strand 
of the other, are probably never exactly alike, as in Eranthis and Anemar- 
rhena . If her figures were drawn under the camera, the size of the xylem 
vessels, expressly concerned in carrying water, are not precisely alike,— 
and such may, perhaps, make all the difference in deciding which blade 
shall be arrested, as in all Monocotyledons. 
However it may have been brought about in the first Monocotyledons 
evolved, there are too many collateral coincidences, speaking broadly, 
between water-effects on Dicotyledons and existing Monocotyledons, in all 
their organs, to gainsay the probabilities that water was, in some unknown 
way, the primary cause of the monocotyledonous condition. 
Monocotyledons were probably first evolved in the early part of the 
Secondary Epoch as soon as Angiosperms were established from Gymno- 
sperms; and as we do not even know what the first dicotyledonous 
Angiosperms were like, one cannot avoid speculating as to the process ; 
for no existing order of Monocotyledons can be pronounced primitive with 
any degree of safety ; because degradations are misleading, if any be 
selected as such, from being of a more simple type. 
18. Non-inheritance, Imperfect and Complete Inheritance, 
of Acquired Characters. 
Since external conditions may graduate one into another, so that 
mesophytes are intermediate between xerophytes and hygrophytes, so, 
too, the same species may find it necessary to adapt itself to soils, &c., of 
more than one kind. To do this the plant retains the power of response to 
either condition, and the characters acquired in one are not necessarily 
hereditary in the other, as in the case mentioned of normally amphibious 
plants. 
To become hereditary under any change of environment, it is necessary, 
as a rule, for the species to have lived for many generations under the same 
conditions as those which produced its special characteristics. 
A partial inheritance may last for one or two years ; just enough to 
