HEREDITY AND EVOLUTION 269 
appeared late in geological history. The successive ages are fre- 
quently called the Age of Invertebrates, the Age of Fishes, the 
Age of Amphibians, the Age of Reptiles, and the Age of Mam- 
mals. These great groups of animals appeared in the above order 
and were dominant in the ages which bear their names. Animals 
of the early ages, such as gigantic amphibians and reptiles and 
the birdlike reptiles, have disappeared, and many of them have 
left no descendants or close relatives. 
Incomplete as a fossil record is, it leaves no doubt that the 
plants and animals of one age were derived from those of a pre- 
vious age by gradual changes, and that the accumulation of these 
changes through the long periods of geological time has resulted 
in an evolution from very simple ancestors to complex organisms. 
Course of evolution. From what has been said about the causes 
of evolution it will be evident that evolution does not follow 
a straight line but that it rather pursues a branching course. 
A diagrammatic representation of the course of evolution is 
therefore drawn as we should draw the branches of a tree, rather 
than as a straight line. Because one form has given rise to a 
second form it does not follow that the first form will continue 
or ever again give rise to the second form. The evidence from 
geographical distribution supports the idea that the evolution 
of a given form will produce different results in different times 
and places, as, when two regions with similar floras and faunas 
become separated, their floras and faunas develop along different 
lines and the differences increase with the length of time the 
two areas are separated. We sometimes hear the question, 
If lower forms gave rise to certain higher forms in the past, 
why do they not continue to do so? In the first place, the 
ancestors of the higher forms are, for the most part, dead. In 
the second place, the evolution from one form to a very different 
form requires the combination of a great many circumstances 
acting through a long period of time, and it is not to be expected 
that all these conditions will ever exist more than once. 
In view of the above, what is the relation between the simpler 
and the more complex plants that exist today ? This question 
