"We learned that a virus freshly isolated from man or animal did not necessarily behave like the virus strain that we maintained in the laboratory, so-called tame virus, which might have been passaged through animals or tissue culture for generations. Virologists had to recognize that by transmission of a virus from animal to animal or from culture to culture over a period of time, what we ended up with through a process of natural selection or genetic selection was a virus adapted to growth in a new environment. A population of viruses is no more homogeneous than the human population; by passage, one culls out all of the inhomogeneous particles and leaves behind those which survive and grow so well,"