THEORIES OF EVOLUTION

1. Lamarckism ( Theory Of Inheritance Of Acquired Characters) : The theory of evolution was first forwarded by a renowned French naturalist Jean Baptiste De Lamarck (1744-1829). In 1809 Lamarck published Philosophie Zoologique. In which he have explained the changes that occur in the formation of new type. The theory of Lamarck are as follows -

i.Tendency to grow : All the organisms possess the tendency to grow. The internal forces of life tend to increase the size of the parts of organism up to certain limit and the limit of growth is determined by life itself.

ii. Formation of new organs : The formation of new organs or a body part occurs due to the need and want.

iii. Use and disuse of organs : The organic evolution of plants and animals took place by the influence of the environment according to Lamarck. The gradual change of the environment needs change in animals according to the demands on certain organs. Organs which are used more extensively would enlarge and become more efficient and the organs which are not used degenerate and atrophy.

iv. Inheritance of acquired characters : The characters which are acquired during the life time of individual are transmitted to next generation into new species totally different from the original one. New species are evolved after many generations according to Lamarck by acquiring new characters or losing old ones.

Examples: In ducks, swans and other water birds webbed feet developed by constant stretching of the skin of their feet while swimming on the water surface. Giraffes obtained their long necks and tall feet by stretching their neck to browse upon trees as it needed greater height to reach the higher leaves and in the process of reaching leaves necks became longer in successive generations modern giraffe were result.The limbs are absent in snakes and other burrowing animals because there are no use of such organs.

Criticism Of Lamarckism

i. According to Lamarck all organisms have tendency to grow but many plants contradict this principle by showing reduction in size during evolution.

ii. There is no dispute regarding the environment and individual according to the use and disuse of organs but the inheritance of that effect has become a very controversial subject. The theory on which the inheritance of acquired character based was mainly opposed by a German biologist August Weismann (1834-1914). He purposed "Germplasm theory" . According to this theory continuity of inheritance is concern with the germ cell and the somatic cells have nothing to do.

iii. The muscles of the shoulder of blacksmith develops much due to its constant use during hammering but it is not inherited in their offsprings.

iv. The tradition of boaring of ears and nose by women has been continued from centuries but their progeny do not show trace of holes in nose and ears.

Darwin's Theory Of Natural Selection :

An English naturalist Charles Robert Darwin (1809-1882) published his famous book called " The Origin Of Species By Natural Selection" . This theory is based on a mass of accurate observations and prolonged experiments led whole to believe in the doctrine of evolution. Darwin accompanied Adam Sedgewick on an exploration of North Wales. He spent five years (1831-1836) during the Voyage on the ship called Beagle and made extensive Observations on animal and plant life and brought back a lot of explanation for further examination. He found a living laboratory of evolution along the south American Coasts. In Galapagos Island he found many species of given genus confined to a single island. Various islands had different species of finches of different size and he concluded that finches were arrived from the mainland of South America and has undergone profound changes under the different conditions of the individual island. In 1858 Alferd R. Wallace (1823 -1923) also came in same conclusion.

The factors of Darwin's theory of natural selection are

i.