In the opening episode, the Doctor kidnaps Susan’s schoolteachers so the travellers can avoid detection. His ethical boundaries are tested again in the third episode as he makes to bludgeon the injured caveman Za (Derek Newark) thereby unblocking the route to escape, before bluffing, “I was going to get him to draw our way back to the TARDIS.” It is the civilised teachers who intervene to protect Za but their liberal humanism is neatly contextualised by the grim realisation that 
Old Mother (Eileen Way) has been slain by the ambitious, would-be-leader Kal (Jeremy Young). Perhaps it is the intercession of modern morality that finally stirs the tribe people to see beyond the day-to-day scuffle to survive, with all its attendant carnage and bloodshed, and recognise the social value of munificence. Doesn’t Za become the mouthpiece of this philosophy when he later counsels Hur (Alethea Charlton) “Listen to them, they do not kill”? On the other hand, in rescuing Za, don’t the TARDIS crew put themselves back into mortal danger? Beneath our civilised skins lies a dark and ruthless past, one that doesn’t always respond to morality or reason.