B. Galaxies as "Island Universes"
Once upon a time there was a single galaxy. William and Caroline Herschel had drawn a map of the Galaxy (Herschel, 1785) on the basis that the Sun was near the center of the Galaxy, and this image persisted into the 20th Century with the "Kapteyn Universe" (Kapteyn, 1922) which depicted the the Milky Way as a relatively small flattened ellipsoidal system with the Sun at its center, surrounded by a halo of globular clusters. Trumpler (1930) recognized the role played by interstellar absorption; he provided a far larger view of the Galaxy and moved the Sun outwards from the center of the Galaxy to a position some 30,000 light years from the Galactic Center.
Competing with this view was the hypothesis of Island Universes, though at least some astronomers 100 years ago thought that had been completely ruled out. Remember that 100 years ago it was not known that the "nebulae" were extragalactic systems: they were thought of as whirlpools in the interstellar medium.
The controversy between the Great Galaxy and Island Universe views culminated in the great debate between Curtis and Shapley in 1920 (Hoskin, 1976). Shapley, who had earlier placed our Sun in the outer reaches of the Greater Galaxy by observing the distribution of globular clusters 3, defended the Great Galaxy hypothesis and won the day for all the wrong reasons.
However, it was left to Edwin P. Hubble to settle the issue in favour of the Island Universes when he found Cepheid variables in the galaxy NGC6822 and the Andromeda nebula (Hubble, 1925b, 1925a).
There was one anomaly that persisted into the early 1950's: our Galaxy seemed to be the largest in the Universe. This was resolved by Baade who recognized that there were in fact two populations of Cepheid variables (Baade, 1956). This doubled the distances to the external galaxies, thereby solving the problem.
For Hubble and most of his contemporaries what had been found were "field galaxies" largely isolated from one another. This was in part due to the sorts of telescope and their fields of view that Hubble was using (Hubble, 1934, 1936) and also in part due to the lingering effects of the phrase "Island universe" which evoked images of isolation. Indeed, as late as the 1960's, astronomers who should have known better said that galaxies were the building blocks of the Universe (eg: McCrea (1964) and Abell in undergraduate lectures at UCLA 1961-1963).
In fact, most galaxies are clustered. This is implicit in images taken with smaller telescopes having larger fields (Shapley often said that large telescopes were over-rated (Shapley, 1932), perhaps in part because he had deliberately cut himself off from them by moving to Harvard) and explicit in the remarks of Zwicky (1938, 1952) who had begun to look at the Universe through Schmidt-coloured glasses. (The 18" Schmidt telescope on Palomar Mountain came into use a couple of years before).
3 We should recall that at about this time Lindblad (1926) and Oort (1928) showed that the stars in the Galaxy were orbiting about a distant center, thus clearly placing the Sun elsewhere than at the center. Back.