Cracking / perishing tyres.

Tempus Fugit
Tempus Fugit Posts: 19
edited March 2009 in Road beginners
I could do with some advice please - I have a Viner Gladius fitted with Schwalbe Stelvio tyres and having done the total mileage of 350 miles on them , I've noticed that the rubber is perishing / cracking in places. The bike is hung up on hooks in the garage and is never taken out in the wet , so I don't understand why they are degrading so soon.
I have had this problem on my winter bike ( Vredestein Fortezza TriComp tyres ) , but as this bike gets more hammer in crappy conditions , I could understand it!
Any help would be greatly appreciated.