Theory:

Critics against Lamarckism:
Due to the evidence against the theory, Lamarck's theories were subjected to criticism. Some of them are as follows:
  • Lamarck stated that all the acquired characters are of use to the organism. Hence, they are inherited to their next generations. But this was not accepted as all the acquired characters need not be inherited to the next generation. An example of this is the boring of the pinna of the ear or women which is not carried to their next generations.
  • The use and disuse theory has also been criticized due to several observations. Continuous usage of the eyes of the heart, for example, did not result in an increase in size.
  • Weismann's germplasm theory criticized Lamarckism that the characters corresponding to somatic cells cannot be inherited, and only the characters which correspond to the germ cells can be inherited.
  • He provided experimental proof for this theory in which Weismann cut off the tails of rats for 22 generations and allowed them to breed. But their offsprings were again tailed because the cells responsible for the growth of the tail are somatic and are not germ cells.
739pxWeismannexperiment.png
An image illustrating Weismann's germplasm theory  
Neo-Lamarckism:
The term neo refers to new. Neo-Lamarckism is the modified form of Lamarckism. According to this theory,
  • Environment influences organisms and changes their heredity.
  • Not all, but a few of the variations acquired by an organism can be passed on to the offspring.
  • Internal vital force does not play any role in evolution.
  • Only the variations which occur in the germ cells are passed on to the offspring.
  • The characters affecting somatic cells may also be inherited like that of the vegetative propagation in plants.
Differences between Lamarckism and Neo-Lamarckism:
S.No
Lamarckism
Neo-Lamarckism
1.It is the original theory given by Lamarck.It is the modification of the original theory given by Lamarck so that it is more suitable to the modern knowledge.
2.The internal vital force, environmental factors, and the use and disuse of organs are all emphasised in this approach.It does not give importance to any of these factors.
3.According to this theory the change in environment brings a conscious reaction in animals.This theory stresses on the direct effect of changed environment on the organisms.
4.According to Lamarckism the acquired characters are passed on to the next generation.According to this theory only the modifications which occur in the germ cells are transferred to the offsprings.
Reference:
https://commons.wikimedia.org/wiki/File:Weismann_experiment.png