In the course of time, the concept of cluster of galaxy has significantly evolved. A concentration of nebulæ, maybe galactic objects, like a star cluster, in the early days of the XX century. A remarkable (but relatively rare) concentration of external galaxies, which nevertheless were much smaller than our own, in Hubble's times. Or rather the extreme of a continuous clustered distribution of galaxies, according to Carpenter. A stable, bound dynamical system, with an incredible mass, according to Zwicky. Or instead, a light, rapidly disrupting system, whose explosion was powered by unknown mechanisms operating in the centres of its galaxies, according to Ambartsumian. A galaxy incubator, according to Zel'dovich' top-down scenario, or rather an association of free galaxies, according to Peebles' bottom-up scenario. A dangerous place to live, for spiral galaxies, according to nurture scenarios for galaxy evolution. Or maybe a very quiet place, where old ellipticals can passively evolve for billions of years, according to nature scenarios for galaxy formation. A knot in the filamentary structure of the Universe, when the Large Scale Structure was finally unveiled by observations in the 80's. A cluster of gas, rather than a cluster of galaxies, in the 90's, when X-ray surveys became more effective in finding high redshift clusters than the traditional optical methods. And now, finally, a cluster of dark matter, a dark cluster, which will be found through the weak lensing surveys (see MELLIER, these proceedings).
If the evolution of clusters is slow (see, e.g., DICKINSON, these proceedings), not so slow is the evolution of science. Moreover, this evolution is often discontinuous and non-monotonic. Zwicky's missing mass was re-discovered in galaxy halos after 40 years; the existence of significant subclustering in clusters was demonstrated in the 60's by van den Bergh, but the irregular X-ray morphologies of clusters came as a surprise to many astronomers. The Local Supercluster was hinted at by J. Herschel in the XIX century, and rediscovered several times before de Vaucouleurs re-affirmed its existence, in the 50's. And many other examples can be found by reading this review. We certainly need to keep track of the evolution of science, or we risk to forget about fundamental results that might take years to be re-discovered. I hope this modest review can be helpful in this respect.
``Quello che lei non sa è il vero scopo del nostro lavoro [...] È perchè tutto non sia stato inutile, per trasmettere tutto quello che sappiamo ad altri che non sappiamo chi sono nè cosa sanno.''
Italo Calvino, La memoria del mondo
Acknowledgments
This paper is dedicated to my wife Patrizia, who has tolerated me sharing my free time among Abell, Herschel, Zwicky, and herself.
I wish to thank Florence Durret and Daniel Gerbal, for organizing such an interesting, exciting, and memorable (ah, la guinguette sous l'orage!) meeting. I also thank the Scientific Organizing Committee, for giving me the opportunity of preparing this review.
Special thanks to Sandro Bardelli and Renata Longo for sending me copies of W. Herschel's works, and Hubble's Realm of the Nebulæ, respectively. Piotr Flin's remarks have been very useful for the writing of Section 2.1. Stefano Borgani's careful reading of Section 5.1 is gratefully acknowledged. Many thanks also to the librarians of the Trieste Observatory, Laura Abrami and Chiara Doz, who assisted me in my bibliographic research.
Et un grand merci pour tout à Daniel.
This research has made use of NASA's Astrophysics Data System Abstract Service.