I'm finally back from a holiday without computers, and I have a batch of questions to try to answer. Here's the first one, from Le Hai Thanh:
I have a question about the verb following "the variety of..."
"What they share in common is the extraordinary variety of plant and animal life-forms that IS/ARE the necessary part of the on-going process of their formation".
Which verb should I use in this case? IS or ARE?
This is a tricky one!
We have several "collective nouns" that can be either singular or plural, and the article we use in front of them can determine whether they're singular or plural:
A number of students are going on to college.
The number of students in college is larger this year.
In the sentence you ask about, we have to decide if the subject of the clause is "variety" or "life-forms." Is the variety the necessary part of the on-going process? Or are the life-forms the necessary part of the on-going process?
I would go with "life-forms" and "are." The life-forms are part of the on-going process; their variety is interesting, but not really crucial for the process.