290 INTRODUCTION 
paratively well developed. In addition to the many kinds of 
plants now living, many other kinds once existed but are now 
known only by their fossils. 
Among the kinds of plants, including both the living and fossil 
forms, there are almost all degrees of complexity, ranging from 
the simplest unicellular plants to the most highly developed 
Flowering Plants. Although varying widely in complexity, the 
various kinds of plants are evidently related as a study of their 
structures and habits reveals. Scientists believe that the living 
forms have come from previously existing forms and hence are 
related through a common ancestry. They have originated 
through the process known as evolution, which assumes that the 
first plants on earth were extremely simple and from these simple 
forms the more complex forms arose. In response to a changing 
environment or due to changes arising wholly within, the simple 
forms gave rise to more complex forms, which in turn gave rise to 
forms still more complex. Thus through slow changes involving 
millions of years the highly developed forms were evolved. 
Evolution has generally been progressive, giving rise to forms 
with higher organization, greater perfection of parts, and in- 
creased efficiency of function. Sometimes, however, evolution 
has been retrogressive, and forms have been reduced to simpler 
forms, through becoming more simply organized and less efficient 
in function. For example, in this way the Fungi are supposed to 
have arisen from the Algae. Progressive evolution has not been 
direct from the simplest to the highest organisms, but has been 
along many lines which, although usually progressive, have been 
more or less divergent and this accounts for many kinds of 
organisms among both plants and animals. Animals and plants 
can be distinguished in their higher forms on the basis of loco- 
motion, methods of getting food, character of the skeleton, and 
so on, but in their simpler forms animals and plants are not easily 
distinguished, and this fact suggests that plants and animals 
arose as diverging lines from the same preéxisting organism. A 
diagram of evolution in plants looks like a tree with many 
branches. The trunk represents the main line and the branches 
the diverging lines of evolution. The lowest branches with their 
sub-branches represent the groups of the simplest plants and 
their relationships. The groups of Seed Plants and their rela- 
tionships are represented by the topmost branches, and the 
