the idea of the weismann barrier is central to the modern evolutionary synthesis, though it is not expressed today in the same terms. in weismann's opinion the random process of mutation in the gametes (or stem cells which make them) is the only source of change for natural selection to work on. weismann was one of the first biologists to deny the inheritance of acquired characters entirely. weismann's ideas came before the rediscovery of gregor mendel's work. though weismann was cagey about accepting mendelism, younger workers soon made the connection.