Electrostatic Potential
The electrostatic potential at any point defined as the work done in bringing a unit positive charge from infinity to that point. The S.I. unit of electrostatic potential isvolt. It is denoted by symbol V.

Potential Difference
The potential difference between two points A and B in an electric field may be defined as the amount of work done in moving a unit positive charge from point A to point B.

Potential Difference = 

or

V = 
The S.I. unit of potential difference is Volt. Potential difference is a scalar quantity. The potential difference is measured with the help of an instrument called Voltmeter. It should be noted that in order to measure the potential difference of a conductor the voltmeter is connected in parallel circuit with the two ends of the conductor as shown in figure.

1 Volt
The potential difference between two points in an electric field is said to be 1 volt if 1 Joule of work is done in bringing 1 coulomb of positive charge from one point to another.

1 Volt = 

Test Your Understanding and Answer These Questions:

1. What is electrostatic potential?
2. What is potential difference?
3. Define 1 volt?
4. Name the instrument by which potential difference is measured.

Biogenetic law is also known as Recapitulation Theory. It was postulated by Ernst Haeckel in 1866. According to the biogenetic law ontogeny recapitulates phylogeny , i.e. during the development of the animal embryo the evolutionary development of the species is repeated. The theory was influential and much-popular earlier but these days this theory is of little significance.

Test Your Understanding and Answer These Questions:

1. What is biogenetic law?

Different scientists have tried to explain the process of organic evolution. So they have given different theories for the Evolution of organisms. Out of them the names of three scientists are important Carlos Linnaeus, Jean Baptist de Lamarck and Charles Darwin but we shall discuss the two important theories of evolution which are put forward by Lamarck and Darwin.

Lamarck’s Theory of Evolution or Lamarckism
This theory was given by a famous biologist Jean Baptist de Lamarck in 1744, in his book ‘Philosphie Zoologique’.

The main postulates of Lamarck’s theory of evolution are:

1. Effect of the environment
The change in the environment creates new needs in organisms, which results in change of habits of organisms. The new habits involve more use of certain organs of the body and on the other hand less use of other body organs.

2. Use and disuse of organs
The use and disuse of organs by an organism leads to acquisition of new characteristics.

3. Inheritance of acquired characters
The characters acquired by an individual are then transmitted by heredity to the next generation. In this way, in every generation new characters get accumulated and after a number of generations, a new species with modified characters is formed.

Criticism
Lamarck’s theory of evolution was very strongly criticized by another biologist, August Weismann; he showed that even after cutting the tails of rats continuously for 21 generations, a tailless rat was never born.

Test Your Understanding and Answer These Questions:

1. Explain Lamarck’s theory of evolution.
2. Give criticism of Lamarck’s theory of evolution.
3. Name the book in which Lamarck wrote his theory of evolution.

Embryology is the study of the development of the embryo of an animal. It also provides evidence for the evolution of organisms. For example, if we carefully observe the embryos of vertebrate animals like fish, tortoise, chick, rabbit and man. We will find out that all of them resemble to each other in their initial stages of development to such an extent that it becomes difficult to distinguish them from each other.

This resemblance of the embryos of different vertebrates indicates that all of these animals have evolved from a common ancestor.

Test Your Understanding and Answer These Questions:

1. What is embryology?
2. How do embryological studies provide evidence for evolution?

Fossils are the remains of dead bodies of prehistoric plants and animals. These provide good evidence for the organic evolution. Archaeopteryx is its best example. This animal has both the characters of reptiles and birds. Archaeopteryx had feathered wings like those of birds but possessed claws on the wings like reptiles. It also had a beak like birds but also had teeth like reptiles. This suggests that birds have evolved from reptiles.

Test Your Understanding and Answer These Questions:

1. What are fossils?
2. Give examples of fossils.
3. How studies of fossils provide evidence for evolution?

The living animals which possess the characteristics of two different groups of animals are known as connecting links. For example,

Lung fish: – Lungfish possesses the characters of both the fishes and amphibians. It resembles the fishes in having paired fins, gills and scales over body, on the other hand, it also have the ability to respire through lungs which is the characteristic property of amphibians. Thus lung fish acts as a connecting link between fish and amphibians. This suggests that in the past, amphibious organisms have developed from fishes.

Test Your Understanding and Answer These Questions:

1. What is a connecting link?
2. How can you prove that birds have evolved from reptiles?

It is the reappearance of a certain ancestral, not parental organs which has either been completely disappeared or reduced.

Atavism also gives evidence of organic evolution. The best example of atavism is the presence of a short tail in some human babies. Other than this, the presence of moving pinna, very large canines and very long and dense hair in some people are also some more examples of atavism. This shows that the humans have evolved from those organisms which possess a tail, moving pinna, large canines and lone and dense hair on their body.

Test Your Understanding and Answer These Questions:

1. What is atavism?
2. Give one example of atavism.