Behavior of lower organisms

The ability to respond to irritations from the environment - irritability - is the main property of every, even the most unicellular organism.

An already bare protoplasmic mass of amoeba responds to mechanical, thermal, optical, chemical, electrical stimuli (i.e., all the stimuli to which higher animals react). In this case, reactions cannot be directly reduced to the physical action of the stimuli that cause them. External physical and chemical stimuli do not directly determine the response of the organism; the relationship between them is ambiguous: the same external irritation, depending on various circumstances, can cause different and even opposite reactions: both positive - towards the source of irritation, and negative - from it. Consequently, external stimuli do not directly cause a reaction, but only condition it through the internal changes that they cause.

Already here there is a known selection from the environment, some selectivity and activity. Because of this, even the most elementary behavior of the lower organism cannot be reduced to the physicochemical laws of inorganic nature. It is governed by the biological laws, according to which the reactions of the organism are carried out in the sense of adaptation - the main type of biological relationship of any animal organism with the environment.
At all stages of development, the behavior is determined by both external and internal moments, but at different stages of development, the ratio between external, in particular physicochemical, stimuli and internal processes that mediate their influence on behavior is different.

The higher the level of development, the greater the role played by internal conditions. In humans, sometimes an external stimulus turns out to be only an occasional reason for action, which is essentially an expression of a complex internal process: the role of external stimuli in this case is only very indirect. On the contrary, at the lowest levels of organic development, the role of external stimuli is great, so that under certain conditions the reactions are almost more or less uniquely determined by external physicochemical stimuli.