HEREDITY AND EVOLUTION 261 
Evidence of change. Plants and animals at the present time 
undergo mutations, or, in other words, they change. This proves 
at once that organisms can change or undergo evolution. The 
only question that remains is to determine the extent to which 
they may evolve or have evolved. Fossil records, comparative 
morphology, and geographical distribution are probably the most 
important lines of evidence for evolution in plants; and one can 
realize the completeness of this evidence only after making a 
thorough study of these subjects. The evidence for evolution is 
not, however, confined to these, as all the fields of biology afford 
striking evidence. The more one knows of biology the more 
complete the evidence becomes. 
Comparative morphology. A study of the many kinds of “Absit 
and animals shows that in both cases they range from single- 
celled individuals through many gradations to the most complex. 
Moreover, these different forms can be arranged in series which 
indicate that the more complex organisms have been derived 
from simpler ones. Indeed, this is true to such an extent that 
the only logical system of classification takes the form of a tree, 
indicating that the specialized forms came from more primitive 
groups, and these from still simpler ones, all pointing toward 
a common origin for the whole plant kingdom. 
The most complex plants are characterized by having flowers 
and a complicated method of reproduction. Plants show many 
gradations, from single-celled plants without sexuality (Fig. 326) 
to the complex condition found in flowering plants. In some 
simple single-celled plants there is a fusion of similar cells; in 
such cases there is no differentiation of sex. In some slightly 
higher types the fusing cells are alike but are formed by many- 
celled individuals; such plants also have no differentiation of sex. 
Even some still higher types show only an indication of the differ- 
entiation of sex. Between the simplest plants with eggs and sperm 
and the flowering plants there are still many gradations of com- 
plexity in sexuality. This series will be treated in later chapters. 
Just as there are gradations in the development of sexuality, 
so also there are gradations in vegetative complexity. There are 
