As a result, the teleological argument supporting Cuvier’s first diagnostic characteristic, the presence in animals of an alimentary cavity, or internal pocket, in which they can transport their nutriment, has collapsed, at least as far as his way of articulating it goes. Baron Georges Cuvier Regne Animal. And, with the advancement of microscopic anatomy, the fact’s universality among species is no longer predictable. Many creatures, especially those with complex structures, live parasitically within others and have no alimentary cavity.
Innumerable plants and free plant cells are now known to spend the majority of their lives in an actively locomotive state, indistinguishable from that of a simpler animal, and, while in this state, their movements appear to be as spontaneous and the product of volition as those of such animals.
Their food is delivered to them, not only ready-cooked but already digested, and the alimentary canal has been rendered unnecessary. Again, the males of most Rotifers lack a digestive tract; as a German naturalist has observed, they devote themselves exclusively to the “Minnedienst” and are among the few realisations of the Byronic ideal of love. Finally, among the lowest forms of animal life, the speck of gelatinous protoplasm that makes up the entire body has no permanent digestive cavity or mouth but instead takes in food from wherever and digests it all over its body.
Although Cuvier’s leading diagnosis of the animal from the plant will not be tested rigorously, it remains one of the most consistent animal characteristics. And, if we substitute the ability to take solid nutrition into the body and digest it for the possession of an alimentary cavity, the definition will now embrace all creatures, with the exclusion of select parasites and the few and exceptional examples of non-parasitic animals that do not feed. However, the newly modified definition will exclude all common vegetative creatures.
Cuvier almost throws up his second distinguishing characteristic when he confesses that it is lacking in lower species.
The third distinction is based on a fundamentally incorrect understanding of the chemical differences and similarities between the elements of animal and vegetable species, for which Cuvier is not responsible, as it was prevalent among contemporaneous chemists. It is now proven that nitrogen is an essential element of both vegetable and animal life matter and that the latter is chemically as complex as the former. Starchy compounds, cellulose, and sugar, which were once thought to be peculiar to plants, are now proven to be regular and natural products of mammals.
Amylaceous and saccharine substances are widely produced, even by the highest animals; cellulose is common as a constituent of lower animal skeletons; and it is likely that amyloid substances are universally present in animal organisms, albeit not in the precise form of starch.
Furthermore, although it remains true that there is an inverse relation between the green plant in sunshine and the animal, insofar as, under these circumstances, the green plant decomposes carbonic acid and exhales oxygen, while the animal absorbs oxygen and exhales carbonic acid, the exact research of the modern chemical investigators of the physiological processes of plants has clearly demonstrated the fallacy of attempting to draw any general distinction between them.
In fact, the distinction evaporates with sunlight, even in the case of the green plant, which, in the dark, consumes oxygen and emits carbonic acid like any other animal.30 On the other hand, plants that lack chlorophyll and are not green are always in the exact same position as animals in terms of respiration. They take in oxygen and produce carbonic acid.
Thus, as knowledge has advanced, Cuvier’s fourth distinction between animal and plant has been utterly discredited, as have the third and second; even the first can only be preserved in a modified form and with exceptions. But has the advancement of biology simply broken down old distinctions rather than building new ones?
With one qualifier, which will be discussed later, the answer to this question is unquestionably yes. The famous research of Schwann and Schleiden in 1837 and the following years founded the modern science of histology, or that branch of anatomy that deals with the ultimate visible structure of organisms as revealed by the microscope, and, from that day to this, the rapid improvement of methods of investigation and the energy of a host of accurate observers have given greater and greater breadth and firmness to Schwann’s great generalization.
Contractility, the fundamental condition of locomotion, has not only been discovered to exist far more widely among plants than was previously imagined, but, in plants, the act of contraction has been found to be accompanied, as Dr. Burdon Sanderson’s interesting investigations have shown, by a disturbance of the electrical state of the contractile substance, comparable to that which was found by Du Bois Reymond to be a concomitant of the activity of ordinary
Three fragile filaments protrude from the surface of Venus’s fly trap’s bilobed leaf (Dionaea muscipula) at a right angle. Touch one of them with the end of a thin human hair, and the lobes of the leaf immediately close together due to an act of contraction of part of their substance, exactly as the body of a snail compresses into its shell when one of its “horns” is irritated.
The snail’s reflex activity is caused by the presence of a neural system within the animal. A chemical shift occurs in the tentacle nerve, which spreads to the muscles that retract the body, forcing them to contract, resulting in the act of retraction. Of course, the resemblance of the acts does not necessarily imply that the mechanism by which they are carried out is the same; rather, it implies a suspicion of their identity, which must be thoroughly investigated.
The results of recent inquiries into the structure of the nervous system of animals converge towards the conclusion that nerve fibers, which we have hitherto regarded as ultimate elements of nervous tissue, are not such but are simply the visible aggregations of vastly more attenuated filaments, the diameter of which dwindles down to the limits of our present microscopic vision, greatly as these have been extended by modern improvements in the microscope, and that, as a result, even the most primitive living beings could have a neurological system. And the question of whether plants have a nervous system takes on a new dimension, presenting the histologist and physiologist with a challenging challenge that must be approached from a new perspective and using methods that have yet to be created.