Question

In Lamarkicsm we say that use and disuse of organs is valid according to which neck of giraffe has elongated.

But inheritance of acquired characters is not valid.

So how do we clarify that the long neck of giraffe has passed on to its progeny?

Answer 1

Lamark talks about acquiring a trait.
Lamarck believed that the long necks of giraffes evolved as generations of giraffes reached for ever higher leaves.
This of course didnot happen in a single day. It should have take thousands of years.
Let me explain how this process could occur.

First a giraffes had short necks. They needed to eat leaves from higher branches. So, a Giraffe tries to reach it . He tries to reach it every day, till he dies, but he will not be able to reach it, because, organs dont grow like that.
If you try to pull your hand for next 10 years doesnot mean that after 10 years you will have a longer arm at end.
Instead, your body will understand your need.
So, this info is recorded in your gene.
The giraffes next progen will have same neck, not longer ones. This is because genes are not yet changed. If the next progeny tries again to reach tall leaves, slowly, after thousands of generations, slowly, genes will change , to match the need of organism. Slowly, each progeny will have little by little, longer necks.
This continues , until body realises that no longer growth is needed. Then , the growth of neck stops, in next few generations.

So, Its the genes that changes first, not the neck. All our activities are recorded on our genes. when found necessary, these genes change and a new character comes up, in this case its the neck.
As genes change first, it is sure that it will be transferred to next progeny.