INTRODUCTION 15 
must therefore look to other sources of information in 
our quest for the ancestors of the present flora of the 
earth. 
Much can be learned as to the relationships of plants 
from a study of their external structure, and the classi- 
fication, especially of the higher plants, is based largely 
upon purely external characters. While such charac- 
ters are usually reliable when dealing with nearly re- 
lated forms, they are likely to be misleading when we 
try to trace out the affinities of plants whose kinship 
is not so obvious. Here it is important to take into 
account, for comparison, the more obscure points of 
structure, —for it not infrequently happens that re- 
semblances,may thus be traced which are not evident 
at first sight. Thus, in comparing the Mosses and 
Ferns, it is the minute reproductive organs and em- 
bryos which show the unmistakable relationship of 
these plants, while their more conspicuous external 
structures are very different. 
There is little question that, as in the study of animal 
forms, it is the careful investigation of the life-history 
of the plant which affords the surest clue to its affinities 
with other forms. The generally accepted view that 
in animals the developing germ repeats in a general 
way the evolution of the race, is also applicable, in 
some degree at least, to plants, and by far the most 
important discoveries, with reference to the origin of 
vlant forms, have been due to studies of this nature. 
Very often the early stages of the embryo and repro- 
ductive organs in different plants reveal resemblances, 
while the adult stages may have, apparently, very little 
in common. These embryonic phases are less affected 
