264 A TEXTBOOK OF GENERAL BOTANY 
which the two areas have been separated from each other by 
barriers such as high mountains or seas. This is easily explained 
by the teachings of evolution. When there is communication | 
between two contiguous areas, the plants and animals of the 
two areas are naturally interchanged. If later the two areas 
become separated, as by the submergence of land connections, 
the flora and fauna of each will continue their evolution sepa- 
rately. Owing to the method by which evolution takes place 
it is not to be expected that the course of evolution will be 
- the same in the two different localities, but rather that evolu- 
tion will take place in a different direction in each. In this way 
the longer the two areas are separated the more distinctly dif- 
ferent their faunas and floras become. Australia has been sepa- 
rated from the rest of the land areas of the world for long ages, 
and as a result its fauna and flora are strikingly different from 
those of the rest of the world. The great development of euca- 
lypts. (fig. 241) and marsupials in Australia is well known. 
Geological evidence. ‘This evidence is afforded by fossils. A 
fossil may be defined as any impression, remains, or trace of a 
plant or animal of a past geological age. By the study of fossils 
we learn much about the floras and faunas of past ages. 
Nature of fossils. The most important types are impressions 
and petrified structures. We are all familiar with the impres- 
sions made by leaves in mud. Plants and animals are frequently 
buried and leave impressions. I? these impressions remain after 
the material in which they are made is turned to rock, the result 
is a fossil. From such impressions (Figs. 242, 243) we can learn 
much concerning the form and shape of organisms or organs, 
but usually nothing of their internal structure. Among the most 
instructive fossils are those produced by petrifaction. In such 
cases the plant or animal material is replaced by stone, and it ig 
frequently possible to study much of the microscopic structures 
of these fossils. In addition to the above classes of fossils we 
sometimes find the original material of organisms preserved. 
Formation of fossils. The rocks of the earth’s surface are 
divided into two classes, according to their origin: igneous and 
