There are instances where the fossil record is not complete

There are instances where the fossil record is not complete which means there are gaps.  Someone comes to you and states that because there are gaps in the record for species ‘A’ this shows that evolution is not true.  How do you argue against this statement?  In other words how do you argue in favor of evolution when gaps are present?  Or do these gaps invalidate evolution?  Restrict your discussion to the fossil record only.