1.   A major difficulty arises when fossil species disappear for good as the physical environment, over millions of years, inevitably changes.

2.   A major difficulty the divers faced was caused by a lack of valves.

3.   Given this, determining the nature of the interactions between the variables becomes a matter of major difficulty.

4.   The major difficulties are overcrowding, lack of books and materials, and low teacher morale.

5.   This might suggest that the major difficulty with closed class items is that they are harder to access than open class items.

6.   Two other major difficulties need special consideration.

7.   This highlights the second major difficulty with the theories, the general assumption that there is a unitary dimension of arousal.