Ferroseed does evolve in Ferrothorn at level 40.
Ferroseed evolves at level 45
Lv 45
Ferroseed will evolve into Ferrothorn.
Level 40 ;-)
You train it to level 40
Train it to level 40.
Ferroseed evolves into Ferrothorn at level 40.
Through leveling up; Ferroseed will evolve into Ferrothorn at level 40.
In Pokémon White, Ferroseed will evolve at Level 40.
Ferroseed evolves into Ferrothorn at level 40.
Ferroseed evolves at level 40