HISTORY OF THE PLANT KINGDOM 299 
as in zodlogy, it is a well-recognized law that the develop- 
ment of every individual usually follows in outline the course 
of development of its group. During the process of hatch- 
ing, while the young animal in the egg is beginning to 
develop into a turtle, 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 this same way, we may, for example, feel 
sure from the resemblance in essentials between the pro- 
thallia of ferns and horsetails that these two kinds of 
plants, so different in the general form and structure of 
the full-grown 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 con- 
dition. By comparing in this way the successive steps in 
the development of great numbers of plants of different 
groups, it has become possible to draw up a sort of pedi- 
gree of the plant world. This is not as yet by any means 
complete, but what is already known on the subject throws 
much light on the reasons for the existence of structures 
in the early part of the life histories of many plants which 
would otherwise seem to be wholly useless and without 
meaning. 
380. Plants form an Ascending Series. — All modern 
systems of classification group plants in such a way as 
to show a succession of steps, often irregular and broken, 
1 See Bergen and Davis’ Principles of Botany, pp. 248, 273, 404, 405. 
