The reference to the forest changing people comes from towards the beginning of this novella, when Marlowe imagines what it would have been like for Roman soldiers to leave their civilisation and to suddenly have to cope with life in very different circumstances: in a barbaric outpost of the Roman Empire, coping with lawless savages and freezing conditions. Note how he refers to how this might have impacted such individuals who were struggling to adjust to their new location:
Land in a swamp, march through the woods, and in some inland post feel the savagery, the utter savagery, had closed round him--all that mysterious life of the wilderness that stirs in the forest, in the jungles, in the hearts of wild men. There's no such initiation either into such mysteries. He has to live in the midst of the incomprehensible, which is also detestable.
The word "savagery" is repeated twice to emphasise the impact that these surroundings would have had on such soldiers, living so far from home. Marlowe likens it to living in "the midst of the incomprehensible." It is no doubt therefore that living under such radically different conditions, away from what elsewhere Marlowe describes as the conditions of civilisation that keep men in check, would change and impact such individuals. The parallel is created between Roman soldiers coming to Britain and also colonial Europeans going to the heart of Africa. Both are shown to be very similar in terms of the impact that the "utter savagery" of the wilderness has on them in terms of the ability and freedom to do anything they want to and to give in to the savagery that they see in their surroundings.
In Heart of Darkness, Marlow repeatedly refers to virtue and civilized behavior as constructed things subject to environmental influences. Civilized people are only civilized because they live in places that force them to be so. After explaining how the "darkness" of the Congo environment has claimed Kurtz, Marlow tells his crew:
"You can't understand? How could you- with solid pavement under your feet, surrounded by kind neighbours ready to cheer you or to fall on you, stepping delicately between the butcher and the policeman, in the holy terror of scandal and gallows and lunatic asylums- how can you imagine what particular region of the first ages a man's untrammelled feet may take him into by the way of solitude- utter solitude without a policeman- by the way of silence- utter silence , where no warning voice of a kind neighbor can be heard whispering of public opinion. These little things make all the great difference. When they are gone you must fall back on your own innate strength, upon your own capacity for faithfulness."
It is only because his crew are used to living in a world of enforced order that they could not contemplate going down the same road Kurtz has walked. The butcher must face animals and get blood on his hands, but nothing but conquered remains reach the people. The policeman can be relied upon to fight all of their battles while they stay safe in their homes, and people who go beyond the bounds of ordered society could end up in prison or in the asylum. There is no call to fend for themselves or rely on their own strengths in a place where there are all of these restrictions on their actions. Without these trappings, they would be alone and forced to resist temptations and ignore the savagery in their natures based on their own merits. Whether it is the forest pressing in on the Romans who first arrived in Britain or the vast jungle of the Congo creeping into Kurtz's soul, people will turn savage more often than not as soon as they abandon towns for the wilderness.