256 ELEMENTS OF BOTANY 
preserved in the rocks. Great numbers of fossil plants 
have been discovered in various parts of the world, but 
there is reason to suppose that all of these are compara- 
tively late and highly developed descendants of the first 
plants that appeared. 
332. Evidence from the Life Histories of Plants. — Every 
individual phanerogamic plant and every one of the higher 
eryptogams during its hfe history goes through a series 
of changes, — from the spore with which it begins to the 
most highly developed form of which that plant is capable. 
This gradual unfolding of organs, from a very simple 
spore as the starting point, means everything to the 
botanist. For in botany, as in zodlogy, it is a well recog- 
nized law that the development of every individual follows 
in outline the course of development of its group. During 
the process of hatching, while the young animal in the 
ege is beginning to develop into a turtie, an alligator, or 
a bird, the general form of the embryo is for some time 
much the same in all three cases. It is probable that this 
arises from the fact that turtles, alligators, and birds have 
sprung from a common ancestor, — an animal which lived 
in the far remote past and which united in its organization 
some of the characteristics of these its descendants. 
Reasoning in the same way, we may feel sure from the 
resemblance in essentials between the prothallia of ferns 
and horse-tails (Figs. 172, 175) that these two kinds of 
plants, so different in the general form and structure of the 
full-crown sporophytes, have a common ancestry. This is 
only one rather simple instance, out of many, of likeness in 
the early stages in the life history of two classes of plants 
which are most unlike in their adult condition. By com- 
paring in this way the successive steps in the development 
