Yes, I appreciated your answers which well hit my questions. (esp the
perfect model parts).

About the plot part, one more question.
Is it possible to make the two plots (northern and southern richness)
sharing the same Y axis (latitude = 0, the equator) ?
In other words, southern richness would be on the left side of the Y axis,
while northern one on the right side.

Elaine


Two plot panels on the same device like:

layout(1:2)
plot(1:100)
plot(1:100)
layout(1)

=>  Yes that's what I want

The second panel for southern species richness, do you mean you want the
plot to go like this:

plot(1:100, xlim = c(100,1))

=>  Yes, too.

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