Evolution is defined as the slow, continuous, progressive and an irreversible process in which the simplest and primitive organism existed in the past were changed into the advanced and complex organisms of present over a long time period.

The evolution of external features of organisms was accompanied by physiological and biochemical activities. The basic physiological and biochemical activities (respiration, photosynthesis, structure of protoplasm, chemical composition of enzymes and digestive juices) in various organisms are similar which are known as Biochemical evidence of evolution.

Homologous organs	Analogous organs
They have common origin and are built on same fundamental pattern and perform different functions.	They are different in basic structure and origin but serve same function.
Presence of homologous organs in different organisms suggest that these animals are evolved from the same ancestors.	Presence of analogous organs suggest that animals from different ancestral backgrounds can evolve to perform the same function.
Example: The forelimb of vertebrate...

The reptilian characteristics of Archaeopteryx are as follows:

1. Heavy jaw and teeth

2. Presence of claws on its wings

3. Scales like reptiles

4. Tail vertebrae

The avian characteristics of Archaeopteryx are as follows:

1. Presence of beak

2. Forelimbs were modified into wings

3. Body covered with feathers

4. Feathery tail

Recapitulation theory was put forward by E. Haeckel.

Biogenetic law states that ontogeny repeats phylogeny.

Every organism during its development repeats the evolutionary history of its race.

The embryos of the higher animals show resemblances with the adult of lower animals.

Example:

• The life history of a frog clearly shows the phylogenetic history. Frog develops through larval stage similar to that of fish.

• Man develops through a number of embryonic stages similar to that of the lower animals.

...

The importance of paleontology are as follows:

1. Fossils explain the evolutionary history of the organisms. The phylogeny of humans and elephant has also been made possible through fossil records.

2. It has proved that in the beginning unicellular protozoans were formed, from which multicellular animals evolved.

3. It helps to make geological time scale.

4. It provides records of connecting links between two groups of organisms. Example: Archaeopteryx is the connecting link between reptiles and...

Comparative serology is done to determine and confirm the natural relationship of animals due to similarity of blood proteins. The relative degree of precipitation of serum from one group of animals will give idea about closeness between them. The serum of closely related animals is unable to cause precipitation, whereas those of distantly related animals causes highest precipitation.

Atavism or reversion is the anatomical process of sudden appearance or refunctioning of some ancestral organ which were either completely disappeared or present as vestigial organ in the recent parental stocks.

Some atavistic organs occurring in human offspring are as follows:

1. Human baby with a small tail

2. Development of power of moving ear pinnae in some people

3. Long and dense body hair (lion boy)

4. Large sized canine teeth

5. Additional pair of nipple