260 A TEXTBOOK OF GENERAL BOTANY 
eruptions also modify the geography of a locality. Changes such 
as those mentioned have evolved the present configuration of 
the earth’s crust. 
Plants and animals also undergo change; and a study of the 
origin of cultivated plants shows that most of the varieties have 
been produced by the selection of desirable variations. The wild 
species that inhabit the earth at present are also derived from 
previously existing plants. Such species are continually under 
the influence of natural selection. When a favorable mutation 
occurs in either a plant or an animal, there is a tendency for 
individuals having this mutation to persist at the expense of 
those without it. In this way the flora and fauna of the earth 
are gradually undergoing changes which tend to fit them better 
to their environment. ‘This process is naturally a slow one. 
Animals and plants are already very thoroughly fitted to their 
environment, and it is not to be expected that changes which 
would make them more so would be conspicuous during the 
lifetime of one man. 
The evolution of plants and animals, or the accumulation of 
the gradual changes which occur in them, is known as organic 
evolution. 
Organic evolution. According to the theory, or law, of organic 
evolution the present floras and faunas of the world have been 
derived from those of past ages by gradual changes. During the 
course of these changes the organisms have, in general, become 
more complex and better fitted to their environment. The 
gradual process of evolution has, moreover, evolved plants and — 
animals that are suited to very diverse environments. The orig- 
inal ancestors of present-day plants and animals must have been 
very simple indeed as compared with the most complex of their 
modern descendants. 
The general similarity of protoplasm in physical structure, 
chemical composition, and physiological responses certainly indi- 
cates that all protoplasm came from the same source. Moreover, 
the similarity of the phenomena of sexuality and inheritance 
argues for a relationship of all higher animals and plants. 
