 A second point of scientific disagreement concerned the evolutionary origin of instincts. The prevailing view, held by Romanes and others, involved the inheritance of acquired habits. “Late-19th century psychologists and intellectuals in general assumed that heredity and environment could not be separated functionally or treated as independent factors, because change induced by the environment would sooner or later (depending on the author) become a part of the organism’s racial heritage” (Cravens and Burnham 1971, 637–38). This model was fiercely criticized by August Weismann (1891 (1888)), who drew a sharp distinction between germ plasm and soma. On Weismann’s model, the germ plasm (i.e. hereditary material) is sequestered early in development placing it beyond the influence of somatic modifications, including habits or other traits acquired over the life of an organism. In addition, Weismann maintained that selection acts only on traits that emanate directly from the germline (Johnston 1987). He allowed that some traits are the product of environmental modifications during development. But these, Weismann held, could never become adaptations. Thus, Weismann distinguished hereditary traits, which were under direct control of the germline and could become adapted to the environment, from acquired traits, which were due to arbitrary environmental influences on the soma.