The Mendelists said traits didn't change, they just got shuffled into new combinations. The Darwinists insisted that it must be possible for totally new traits to arise. During the first few years of the century, this conflict created a real schism in the ranks of biologists. This has certainly been true of the concept of evolution, and it was also true for our understanding of heredity. In the second decade of the 20th century, some geneticists began to devote more and more time to the study of mutations.
Mutations are permanent changes in genes.
Eventually, the two sides in this artificial division of the science of biology hammered out their differences, and produced a new and improved version of the theory of evolution which incorporated modern concepts of genetics and finally filled the only significant hole in Darwin's original theory. This modernized version of the theory of evolution is called the New Synthesis, or Neo-Darwinism.