Original: In the late 19th century it came to mean the concept that natural selection was the sole mechanism of evolution , in contrast to Lamarckism , then around 1900 it was eclipsed by Mendelism until the modern evolutionary synthesis unified Darwin 's and Gregor Mendel 's ideas .
Simplified: In the late 19th century it came to mean the concept that natural selection was the only mechanism of evolution , in contrast to Lamarckism .