Charles Robert Darwin was a great naturalist and philosopher of
England. At the young age of 20, he got an opportunity to have a voyage of
exploration on a famous ship called HMS Beagle in 1831 A.D .During his
expedition, he visited many island of Atlantic ocean , some coast of South
America , some island of South Pacific etc for five years. During his visit, he
collected a lot of data on variation.
After five years,
he returned in 1836 A.D and compiled all his data which he had collected during
his visit and published his monument work called "Origin of Species by
Special creation" in 1859 A.D in his famous book "Origin of Species".
His theory is also known as Darwinism or Theory of Special creation.
The proposition of Darwin are as
follows:
1. Law of
Prodigality or Enormous Fertility :
Every organism has
tremendous power to reproduce their off spring. Darwin has observed this in
various species in nature. He has given different examples in support of this :
a. Paramecium : Paramecium reproduces 3-4 times in 48
hour. If the rate of reproduction remained the same and all of them survived
then a single paramecia will produce mass of protoplasm 10000 bigger than the
volume of earth in it's 9000th generation.
b. Salmon : Salmon lays about 28 million eggs in one
season. If all of them survived and they continued to reproduced then single
salmon species will occupy all the sea after few generation.
In this way
organism are reproducing at the . If all of them survived then earth would be
full of organism.
2. Struggle for
existence:
The living organism in nature are reproducing at the geometric
ratio i.e slower and the available food resource is only increasing by
arithmetic ratio i.e slower . So,the available food resource may not fulfill
the demand of increasing population. Hence, there is struggle between the
organism to get suitable food, shelter , bedding ground etc. This is called
struggle for existence.
Darwin has observed three types of
struggle in organism and they are :
a. Intra-specific struggle : It is the struggle between individual of
same species for similar need.It is passive. For eg : Two dogs fighting for a
piece of meat.
b.Inter-specific struggle : It is the struggle between organism
of different species which are active. For eg : A cat chasing a rat and the rat
trying to escape from cat.
c. Environmental struggle : It is the struggle of organism against
the environmental factors like excess heat or cold,rain , natural calamities
like volcano, landslide , earthquake etc.
3. Survival of
the fittest / Natural Selection :
During struggle for existence, those organism which posses useful
variation can better can better adapt in the environment can survive, overcome
other where as the other who don't posses eliminate. Those who survive are
called fittest. So it is called as survival of the fittest named by Herbert
Spencer.
Nature is the superpower. It can select organism which can survive and reject which cannot survive. This selection is done by nature so called as natural selection named by darwin.
4. Variation /
heredity
The competition between organism causes them to develop certain characters which help them to survive which is called as variation.
Useful variation make organism fit for struggle and they survive where as useless variation make organism unfit for struggle and they disappear. They are two type :
a. Continuous variation : It is the minor changes adapted by organism with environment. It plays an important role in evolution.
b. Discontinuous variation : It is sudden chromosomal changes in gene of an organism.
The useful variation are transmitted from one generation to another which is called as heredity.
5. Origin of new new-species
Discontinuous variation helps organism to better adapt in the
environment. Useful variation are transmitted from generation to generation.
After some generation , new offspring will be formed which is totally different
from original parent. Hence, there is origin of new species.
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