This is referred to as the Weismann barrier.[2] This idea, if true, rules out the inheritance of acquired characteristics as proposed by Jean-Baptiste Lamarck and implied by Charles Darwin's pangenesis theory of inheritance.[3] However, a careful reading of Weismann's work over the span of his entire career shows that he had more nuanced views, insisting, like Darwin, that a variable environment was necessary to cause variation in the hereditary material.[4]