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Lamarck’s Theory :-
“A Species descended from another ancestral species. Changes occur due to using some organs or not using them. The organs in use are strengthened while those neglected diminish and disappear. The change passes to the following offspring”.
1. The giraffe ancestors were short-necked & they lived on grassland. Dryness killed plants. Short-necked giraffes perished while others which stretched their necks to obtain food from high branches remain. Their necks became taller, a character they passed to offspring.
2. 
Whales used their tail fins & neglected their hind limbs which disappeared.
3. 
Snakes lived in narrow caves where they moved by creeping. They neglected using their limbs which disappeared . Changes were inherited through the different generations .
Criticism of Lamarck’s theory
1- The well developed muscles of a worker due to manual work do not pass to his children.
2- Weismann carried out an experiment where he continued cutting rat tails after they were born for 19 generations but in each case, a rat was born with a tail.
Darwin’s Theory:-
“Darwin attributed evolution to natural selection:
Weak individuals that cannot adapt to the environmental conditions become extinct. Others which have the suitable characteristics to adapt the environmental conditions survive and continue on passing these characteristics to their offspring. Continuous selection causes the change of different species” .
The length of the Giraffe’s neck according to Darwin evolved by natural selection while in Lamarck’s is due to law of use. Darwin explains that the giraffe’s ancestors were short-necked but few had long necks which according to natural selection were able to get the leaves of trees easily , while the short-necked died. The present species with long-necks evolved through natural selection over many generations .