He thought that at some point of time in the history of the size of giraffe was equal to that of deer.Due to shortage of Food material on the ground and to reach the lower branches of trees giraffes started stretching their necks.Because of continuous stretching of neck,after several generations developed necks.Such characters that are developed during the life time of an organism are called 'acquired characters'
Lamarck proposed that these are characters are passed on to its offspring's i.e,to next generation and proposed the theory of ''INHERITANCE OF ACQUIRED CHARACTERS''