Calling shell functions with xargs

I am trying to use xargs to call a more complex function in parallel.

#!/bin/bash
echo_var(){
    echo $1
    return 0
}
seq -f "n%04g" 1 100 |xargs -n 1 -P 10 -i echo_var {} 
exit 0

This returns the error

xargs: echo_var: No such file or directory

Any ideas on how I can use xargs to accomplish this, or any other solution(s) would be welcome.