For the rest，all these gradations，these differences，do but affect the surface of the building.Art has changed its skin，but the actual conformation of the Christian Church has remained untouched.It has ever the same internal structure，the same logical disposition of the parts.Be the sculptured and decorated envelope of a cathedral as it will，underneath，at least，as germ or rudiment，we invariably find the Roman basilica.It develops itself unswervingly on this foundation and following the same rules.There are invariably two naves crossing each other at right angles，the upper end of which，rounded off in a half circle，forms the choir；there are always two lower-pitched side-aisles for the processions—the chapels—sort of lateral passages communicating with the nave by its intercolumnar spaces.These conditions once fulfilled，the number of chapels，doorways，steeples，spires，may be varied to infinity，according to the fancy of the age，the nation，or the art.The proper observances of worship once provided for and insured，architecture is free to do as she pleases.Statues，stained glass，rose-windows，arabesques，flutings，capitals，bas-reliefs—all these flowers of fancy she distributes as best suits her particular scheme of the moment.Hence the prodigious variety in the exterior of these edifices，in the underlying structure of which there rules so much order and uniformity.The trunk of the tree is unchanging；its vegetation only is variable.
1 Histoire Gallicane，Book ii，period ii，fol.130，p.4.—Author's note.
2 This is also known，according to situation，race，or style，as Lombard，Saxon，or Byzantine：four sister and parallel architectures，each having its own peculiar characteristics，but all deriving from the same principle—the circular arch.Facies non omnibus una，non diversa tamen，qualem，etc.—Author's Note.
3 This part of the spire，which was of timber，was destroyed by lightning in 1823.—Author's Note.
Chapter 2-A Vird's-eye View of Paris
We have endeavoured to restore for the reader this admirable Cathedral of Notre-Dame.We have briefly enumerated most of the beauties it possessed in the fifteenth century，though lost to it now；but we have omitted the chief one—the view of Paris as it then appeared from the summits of the towers.
When，after long gropings up the dark perpendicular stair-case which pierces the thick walls of the steeple towers，one emerged at last unexpectedly on to one of the two high platforms inundated with light and air，it was in truth a marvellous picture spread out before you on every side；a spectacle sui generis of which those of our readers can best form an idea who have had the good fortune to see a purely Gothic city，complete and homogeneous，of which there are still a few remaining，such as Nuremberg in Bavaria，Vittoria in Spain，or even smaller specimens，provided they are well-preserved，like Vitré in Brittany and Nordhausen in Prussia.
The Paris of that day，the Paris of the fifteenth century，was already a giant city.We Parisians in general are mistaken as to the amount of ground we imagine we have gained since then.Paris，since the time of Louis XI，has not increased by much more than a third；and，truth to tell，has lost far more in beauty than ever it has gained in size.
Paris first saw the light on that ancient island in the Seine，the Cité，which has，in fact，the form of a cradle.The strand of this island was its first enclosure，the Seine its first moat.
For several centuries Paris remained an island，with two bridges，one north，the other south，and two bridge heads，which were at once its gates and its fortresses：the Grand-Chatelet on the right bank，the Petit-Chatelet on the left.Then，after the kings of the first generation，Paris，finding itself too cramped on its island home，where it no longer had room to turn round，crossed the river；whereupon，beyond each of the bridge-fortresses，a first circle of walls and towers began to enclose pieces of the land on either side of the Seine.Of this ancient wall some vestiges were still standing in the last century；to-day，nothing is left but the memory，and here and there a tradition，such as the Baudets or Baudoyer Gate—porta bagauda.
By degrees the flood of dwellings，constantly pressing forward from the heart of the city，overflows，saps，eats away，and finally swallows up this enclosure.Philip Augustus makes a fresh line of circumvallation，and immures Paris within a chain of massive and lofty towers.For upward of a century the houses press upon one another，accumulate，and rise in this basin like water in a reservoir.They begin to burrow deeper in the ground，they pile storey upon storey，they climb one upon another，they shoot up in height like all compressed growth，and each strives to raise its head above its neighbour for a breath of air.The streets grow ever deeper and narrower，every open space fills up and disappears，till，finally，the houses overleap the wall of Philip Augustus，and spread themselves joyfully over the country like escaped prisoners，without plan or system，gathering themselves together in knots，cutting slices out of the surrounding fields for gardens，taking plenty of elbow-room.