Engels quotes Duhring's statement that,
“In nature, too, one simple type is the basis of all organisms, from the lowest to the highest”, and this type is “fully and completely present in its general essence even in the most subordinate impulse of the most undeveloped plant”. (p 98)
This is also wrong, Engels note. The most simple form of organic life upon which all the higher organisms are based, is the cell, but, among the lower organisms, there are many that stand below the cell, such as the protamoeba, consisting of a single globule of protein, “and a whole series of other monera and all bladder seaweeds (Siphoneae). All of these are linked with the higher organisms only by the fact that their essential component is albumen and that they consequently perform functions of albumen, i.e., live and die.” (p 98) Non-cellular organisms are considered part of a common universal ancestor of all life.
Duhring says that a dividing line between plants and animals is sensation, which requires some central nervous system. Plants are devoid of such sensation. Again, science has shown that to be wrong. But, even at the time Engels was writing, it was clearly wrong for some plants. For example, the carnivorous plants responded to the sensation of insects landing on them to close up and trap the insect before digesting it. Duhring had made this distinction at the same time as noting the existence of intermediate forms that could not be definitively classified as either plant or animal. Engels notes,
“That these intermediate forms exist; that there are organisms of which we cannot say flatly that they are plants or animals; that therefore we are wholly unable to draw a sharp border line between plant and animal — it is precisely this fact that makes it a logical necessity for Herr Dühring to establish a criterion of differentiation which in the same breath he admits is unsound!” ( p 99)
Moreover, the idea that sensation requires the existence of a nervous system is wrong. Plants whose chemical composition and operation of photosynthesis do not require a nervous system to experience the sensation of ultra-violet light that causes them to grow towards the sun, for example. This kind of sensation, towards light, is what leads, also, to the development of photo-sensitive cells that come to form the eye of animals.
Engels quotes Duhring's gibberish definition of life.
“The metabolism which is carried out through a plastically creating schematisation” (what in the world can that be?) “always remains a distinguishing characteristic of the real life process” (p 100)
Engels, then, sets out his own definition. To define life as an organic metabolism is to define life as life, “for organic metabolism or metabolism with plastically creating schematisation is precisely a phrase which in its turn itself needs explanation through life, explanation through the distinction between the organic and the inorganic, that is, that which is living lives and that which is not living. This explanation therefore does not get us any further.” (p 101)
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