This factor can be referred to as an evolution. The first factor is impermanent and repeating, the second factor is slow and non-reversal. In addition, special skills are more subject to changes than individual skills over the time span. With the maturation special skills that pertain to professional and basic levels vary most vigorously. In such a way, the skills of each group reveal at different levels of individual activity.
Individual skills are of interdisciplinary nature and in that sense they are essentially generic.
It should be noted that many skills related to different levels of special skills become a basis for the skills of an individual group. Converse statement is also true.