Evidence For Evolution

The term evolution means an orderly development and is
applied to living things. The theory of evolution says that
plants and animals have changed through generation after
generation and still are changing today. Since this change
has been going on for ages, all things that now live on
earth are much-changed descendants of others that lived
thousands and even millions of years ago. 

The theory of evolution describes changes and tries to
explain them. It shows how these changes followed one
another, and how they produced new kinds of living things
that were far different from their ancestors. Most of these
new types were more complex and efficient than earlier
ones. 

Belief in evolution is based on several different kinds of
evidence. They are variation and change, fossils,
comparative anatomy, and geographic distribution.
 
Variation and Change
 
Many variation pass from one generation to the next without
producing any visible change. Variations that make a plant
resist certain diseases or that help it survive under
unusual conditions, are used by breeders to develop useful
types of plant and domestic animals. Such new types are
examples of evolution on a small and very special scale. 

Fossils
 
Fossils are the remains or traces of things that lived ages
ago. They are preserved in rock layers called strata which
lie one upon another. When fossils are collected form one
layer they reveal variation like those shown by plant and
animal today. When fossils from other strata are studied in
turn, those variation often form series which record the
stages by which new groups developed while old ones
disappeared. 

Comparative Anatomy
 
When different structures of bodies of plants and animals
are compared, relationships and changes can be noted.
Additionally, the origin of structures which now are small
and useless can be explained by this process. For example,
the human appendix is alll that is left of an intestinal
organ which has reached its greatest size and value in
plant-eating animals such as kangaroos and rabbits. 

Geographic Distribution
 
Much evidence of evolution comes from plants and animals
that live on islands far from continents. For example, the
Galapagos Islands have 26 kinds of land birds, all
resembling species found in western South America.
Twenty-three of these species seem to have changed since
they reached the island for the Galapagos birds are
distinct species. They apparently developed there because
of changes that took place after their ancestors drifted
from the mainland of South America. It also means that all
of them must be related.
 
Although general facts and conclusions about evolution seem
clear, the subject still presents many unsolved problems.