A fairly definite conception of the arrangement of the sidereal system evolves naturally from the observational work … We find, in short, that globular clusters, though extensive and massive structures, are but subordinate items in the immensely greater organization which is dimly outlined by their positions. From the new point of view our galactic universe appears as a single, enormous, all-comprehending unit, the extent and form of which seem to be indicated through the dimensions of the widely extended assemblage of globular clusters. The adoption of such an arrangement leaves us with no evidence of a plurality of stellar 'universes.' Even the remotest of recorded globular clusters do not seem to be independent organizations. The hypothesis that spiral nebulae are separate galactic systems now meets with further difficulties. … So long as the diameter of the galactic system was thought to be only a thousand light-years or so, we had a fairly plausible case for the "island universe" hypothesis. But … any external 'universe' must now be compared with a galactic system probably more than three hundred thousand light-years in diameter.
       —Shapley