Scottie, no generation has sold less than the previous (that had to be 2 selling less than 1)
Well, if you count handhelds, there's a change GB was a lot bigger than GBC, making 3rd and 4th gen fairly simmilar, but I doubt it very much.
Roughly sales each gen (without handhelds):
3rd gen: 73M
4th gen: 89M
5th gen: 148M
6th gen: 183M
7th gen: 57M Handhelds each gen:
3+4th gen (GB): roughly 60M (http://vgchartz.com/worldcons.php?date=35309&sort=0)
4th + 5th gen (GBC): roughly 70M
6th gen (GBA): roughly 80M7th gen: 65M,
Edit: what has happened on the site? Things un-para all the time.
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS