The bones of forelimbs of whale, bat, cheetah and man are similar in structure, because

1. one organism has given rise to another 

2. they share a common ancestor 

3. they perform the same function

4. they have biochemical similarities

(b) The bones of forelimbs of whale, bat cheetah and man are similar in structure,. because they have a common ancestor.

These are homologous organs which have different functions across diverse forms, but are developed along same pattern. These organs arise due to divergent evolution.