Selectively Importing Data

First consider the following table:

create table taxon (
taxonid serial,
descr text
);

As I understand it, "serial" means that column will automatically populate
with a numerical key.

If I want to fill the field 'descr' with a list of scientific names stored
in a spreadsheet, then how would I proceed?

I have a spreadsheet that has about a dozen columns. I copied the field
with scientific names into a new spreadsheet. Then I created an empty field
on the left. So I have a spreadsheet with two columns. I saved it as a CSV
file.

When I try to import it with pgAdmin III, I get the error message

WARNING: null value in column "taxonid" violates non-null constraint

How can I import that single column into the second column in this table?
Thanks.