It is designed by Guy Comely of architects HOK. Although HOK is a vast international design firm originating in America, known above all for its stadiums, it put its best home-grown British talent onto this job, and they have risen to the challenge of building in a sensitive historic setting. The Darwin Centre employs a relatively sophisticated architectural language that echoes but never directly mimics the "narrative architecture" of the original 1881 museum by Sir Alfred Waterhouse, which is embellished all over with moulded and carved animals and plants. The idea is to translate the layered richness of the original Romanesque Waterhouse building into a contemporary, more abstract, design. In this it succeeds pretty well. It is absolutely true to its time - terracotta panels, sheer walls of glass supported on steel outriggers, and the inflated transparent foil roof to the atrium are very much the fashionable materials of today. But it sits easily with the Waterhouse - not only the vast bulk of the museum as viewed from Cromwell Road, but also his charming little side entrance lodge close by on Queen's Gate. The Darwin Centre is considerably better than the usual "polite modern", while being better mannered than the prominent early 1970s extension to the museum at the eastern end on Exhibition Road, designed by Government architects.

Once the stock has been transferred, the public will be allowed in for the first time to see this side of the museum's work. Like Kew Gardens' recently-completed Millennium Seedbank down in Sussex, the concept is of a central public exhibition space, from which you can view scientists at work through glass walls, working on such things as biodiversity, pest and disease control. In addition, the more thespian of the museum's staff will come out to meet you, explain what's going on, and take you on tours into the heart of laboratory-land. Interpretation is vital, for otherwise all you see - and these are certainly beautiful and intriguing in a Damien Hirst way - are racks and racks of giant jars containing fish, reptiles, worms, spiders and so on in cocktails of spirit ranging in colour from a pale Sauvignon yellow to a rich Burgundy red. Their eyes goggle at you quizzically. Placed artfully in the Tate Modern, they'd be a sensation. It comes as no surprise to learn that the museum's scientists have advised Hirst on the best ways to pickle his sharks, cows and sheep. But these are continually used for research - particularly those that represent the first examples of their breed ever to be collected. You can refer back to these originals and see evolution in action. Moreover, it is highly likely (though hard to confirm) that many species preserved here are now extinct in the wild. With DNA science advancing rapidly, the collection becomes more valuable all the time.

What happens next? The museum raised the money for the Darwin Centre outright, on its own. But the larger next phase - for entomology and botany, which means 27 million insects and nine million plants - requires Lottery funding, and will be put out to an international architectural competition. Being set back further than the existing additions, one result of the new buildings will be a big courtyard garden here - this is already the only one of our big museums to have gardens around it - but who knows what form the building will take? When the Victoria and Albert Museum ran such a competition a few years back, they went for broke and chose Danny Libeskind's cubist "Spiral" extension, yet to be built. The Natural History Museum will have to make the choice between a "signature" building such as that, and something more reticent, more subservient to the Waterhouse. The Darwin Centre may have arrived almost unnoticed: the design of the next phase will be hard fought and in the full glare of publicity. I can't wait.