Of the contributions made during the essayist period three call for notice: Weismann deserves mention for his useful work in asking for the proof that "acquired characters" or, to speak more precisely, parental experience can really be transmitted to the offspring. The ocurrence of progressive adaptation by transmission of effects of use had seemed so natural to Darwin and his contemporaries that no proof of the physiological reality of the henomenon was thought necessary. Weismann's challenge revealed the utter inadequacy of the evidence on which the beliefs were based. They are doubtless isolated observations which may be interpreted as favouring the belief in these transmissions, but such meagre indications as exist are by general consent admitted to be too slight to be of much assistance in the attempt to understand how the more complex adaptive mechanisms arose.