A few minutes away from the place I am living, there is the Eilenriede forest area, right in the middle of the city. Some part of it is left to its own. No trees are felled there, rotting branches are not cut away, and fallen trees are left to themselves. The area is returning bit by bit to the state of a primeval forest. A few tracks are criss-crossing this areas, narrow, probably unofficial paths.
Following one of these paths, what strikes me is the irregularity of life forms. Our own bodies have shapes determined mainly by our genes, and with many animals it is the same. This might cause us to assume that living things in general have a systematic structure, but going through this small patch of forest shows me that this is not really so. The shape of the ivy plant creeping up a tree follows the shape of the tree trunk and its branches. The shape of the tree itself is quite irregular.
There seem to be three components contributing to the shape or structure of each organism, with varying shares of each in any particular organism. There are aspects of the structure that are completely under the control of the genes. For example, the shapes of the leaves of the trees seem, to a large extent, to be genetically determined. Secondly, there seem to be some random components. For example, the branching patterns of the trees show a lot of irregularity. No two trees look the same. However, this irregularity must be under some genetic control since each species has its typical pattern or statistics. Now that the trees have shed most of their foliage and their branches are clearly visible, I have no difficulty to identify an oak tree or a linden tree, although each of them is different in detail. If I think of my own body, this element of controlled randomness is probably also there, maybe in the exact pattern of the capillaries and the microscopic distribution of cells. In trees, it is just more visible.
The third component consists of those aspects of an organism’s structure or shape that develop in response to its environment. I have already mentioned the shape of the ivy plant that traces the tree or rock it is growing on. The branching of the trees probably also responds to available light (and thus to other trees) and to the prevalent direction of the winds. A tree’s roots will have to respond to other roots, to the stones embedded in the soil and to the availability of minerals and water.
The path is passing along the rotting remains of a tree trunk. Fungi are sprouting from its side. The mycelium, the fine hyph of such a fungus is growing through the tree trunk, maybe following the wood fibers and the yearly layers of wood cylindrically stacked into each other. That network of hyphae of the fungus develops a shape that does not at all depend on its genes, except in its microstructure. If one could edge the wood away and make the mycelium visible on its own, it would trace the shape of its wooden substratum in much detail, a shape that was determined by the genes of the tree, not those of the fungus.
We can see the same in a rotting fruit. A rotting rose hip changes into a mass of bacteria whose shape reproduces that of the former fruit. One could argue that the bacteria are many individual organisms, not one organism, but that is a matter of definition.
What we see in the ivy, in the light-searching branches of trees, in their roots, as well as in the fungus and maybe in the blob of bacteria as well is that the shape these organisms take is, to a large extent, not determined by their genes, but by their environment. The genes equip them with possibilities of reacting to their environment and these reactions then determine the actual forms they take.
Among animals, we see such reactive growth mainly among sedentary forms, like for example, sponges and corals. Those animals that are free floating (or flying) or moving across (or burying into) the ground, on the other hand, seem to lack such adaptive components in their structure. The structure of such animals (and we belong into this group ourselves) seems, to a large extent, to be genetically determined. However, adaptive aspects of the shape are there (even if we do not consider the growth of muscles in response to exercise, and similar changes). We find them inside the nervous system.
Simple animals might just have a limited range of possible reactions to their environment. They don’t learn, so their nervous systems do not change according to the environments. Normally, they are present in large numbers, and if they have to adapt, they do it in terms of that other great learning process, evolution. Evolution however, also does not follow any fixed path. Like the growth of the mycelium and the creeping ivy stalk, it adapts its shape to whatever it finds.
While this evolutionary, genetic adaption also occurs in complex animals, these are able to adapt as well by learning. To some degree, less in some species and more in others, they change the structure of their neuronal networks in response to the environment. As a result, they change their reactions and behaviors. This applies to humans as well, and even to the most extreme extent. Unlike the ivy plant or the fungus in the tree trunk, we do not change our outside shape much in response to our environment, but our brains change, and the results are dramatic.
The sky is gray today and dusk is setting in early this time of the year. It’s time to leave the forest and return home. Due to some little change somewhere in my brain, I manage to find my way back, out of the forest, through the allotments at the edge of the forest, and back to the house where I am living. And due to some little change somewhere in my brain, I am able to write down some of the traces of the thoughts I had during that walk.
(The picture showing the ivy is from https://commons.wikimedia.org/wiki/File:20140227Hedera_helix03.jpg, not from Eilenriede, but I have seen something like that there today. The “reactive structure” of the plant is clearly visible here. The second picture, showing fungus growth on a piece of wood, is from https://commons.wikimedia.org/wiki/File:Fungi_in_Lady_Spencer%27s_Wood_-_geograph.org.uk_-_112004.jpg, also not from where I am, but fine to illustrate my point.)