2. During the course of time, this living substance developed into an unicellular form of life.
3. Then some gradual and orderly changes took place in this unicellular form and it developed into multicellular forms of greater and greater complexity. Then behavioural patterns also gradually underwent many changes.
4. These changes were slow and sequential and took nearly 300 crores of years.
5. These changes and development of living things took place in all aspects and that gave way for many different kinds of living things to come into existence. Hence this process is called the evolution of life on earth, which is also referred to as organic evolution.
6. Organic evolution is thus the progressive development of animals and plants from ancestors of different forms and functions.
7. Organic theory of evolution is the most accepted theory as it answers many questions like how life came into existence, how various forms developed and how more competent species came into existence.
8. Various types of evidences are also available to support the theory. In spite of many adaptive differences in protoplasm and cells, their manifestations of life processes like growth, reproduction, metabolism are essentially the same in all organisms.