The higher manifestations of life are not, as a general rule, possible unless all the extrinsic conditions just mentioned are carried out, and the non-fulfilment of any of them generally causes death; but there are some notable exceptions to this statement. Thus, life may remain in a dormant or "potential" condition for an apparently indefinite time, as exhibited by the great tenacity of life, even under unfavourable circumstances, exhibited by the ova of some animals and the seeds of many plants. A still more striking example of this is afforded by the minute microscopic animals known as the Rotifers or Wheel-animalcules. These little creatures are aquatic in their habits ; and diminutive as they are, they are, nevertheless, comparatively speaking, of a very high grade of organisation. They possess a mouth, masticatory organs, a stomach and alimentary canal, a distinct and well-developed nervous system, a differentiated reproductive apparatus, and even organs of vision. Repeated experiments, however, have shown the remarkable fact that, with their aquatic habits and complex organisation, the Rotifers are capable of submitting to an apparently indefinite deprivation of the necessary conditions of their existence, without thereby losing their vitality. They may be dried and reduced to all appearance to dust, and may be kept in this state for a period of years; nevertheless, the addition of a little water will at any time restore them to their pristine vigour and activity. It follows, therefore, that an organism may be deprived of all power of manifesting any of the phenomena which constitute what we call life, without losing its hold upon the vital forces which belong to it.
* It has not yet been shown that the living matter which we designate by the convenient term of "protoplasm " has universally and in all cases a constant and undeviating chemical composition ; and there is, indeed, reason to believe that this is not the case. It is also certain that there are other materials, the exact use of which we do not at present know, which are absolutely essential to the maintenance of life, probably even in its humblest manifestations.
Again, the vital resistance of the lowest organisms to changes of temperature, seems to be in some cases much wider than that stated above as generally true. Thus, the microscopic organisms known as "vibrios are stated to survive exposure to a temperature of 300o F., and to be wholly unaffected by being frozen; whilst Dallinger and Drysdale have shown that the germs of monads will survive exposure to temperatures of from 280o to 300o F. The presence of oxygen in a free state, too, though essential to the higher forms of life, does not appear to be necessary in the case of some of the lower; since vibrios and bacteria appear to carry on a vigorous life in an atmosphere of carbonic acid gas. Lastly, there are certain conditions, such as the presence of sun-light, which are essential for the maintenance of life as a whole, though by no means necessarily demanded for the life of individuals. Thus, vegetable life is as a whole dependent upon sun-light, and though animals can subsist in darkness, animal life is in reality dependent upon plant-life, so that the total absence of the sun would extinguish all life whatever.
The only other condition of life which need be noticed is the presence of "organisation" in living bodies; and the importance of this has been greatly reduced by the progress of modern science. All the higher forms of life are " organised," - that is to say, they possess distinct parts or "organs," which have certain definite relations to one another, and which discharge certain definite offices or " functions." The protoplasmic and actually living portions even of these, however, appear, under the highest powers of the microscope, to be destitute of any recognisable structure, and are therefore not " organised." Moreover, many of the lowest forms of life (such as the Fora-minifera amongst the Protozoa) fail to fulfil one of the most essential conditions of organisation, being mostly or wholly devoid of definite parts or organs. Nevertheless, they are capable of manifesting all the essential phenomena of life; they are produced from bodies like themselves; they eat, digest, and move, and exhibit distinct sensibility to many external impressions. Furthermore, many of these little masses of structureless jelly possess the power of manufacturing for themselves, of lime, or of the still more intractable flint, external shells of surpassing beauty and mathematical regularity. In the face of these facts we are therefore compelled to come to the conclusion that life is really the cause and not the consequence of organisation; or, in other words, that organisation is not an intrinsic and indispensable condition of vital phenomena. While it is generally admitted that " organisation," in the ordinary acceptation of this term, is not essential to the manifestation of vitality, there are high authorities who consider that the great differences in the vital phenomena of different organisms are due to differences in the " molecular complexity " of the protoplasm forming the bodies of these. Apart, however, from the fact that such differences in "molecular complexity" are, and must remain, hypothetical, this view can hardly be regarded otherwise than as a revival, in another form, of the theory that vital phenomena are the result of the organisation of the living body; since "molecular complexity" is only "organisation," with the "organs" so minute as to evade the highest powers of the microscope.
As to the precise relations which subsist between the " physical basis of life " and the phenomena of vitality, it is held by some that "life" is one of the properties of the albuminous body which we term protoplasm. On this view, life is a form of energy or motion, due simply to molecular movements taking place in the ultimate molecules of the protoplasm, and capable of correlation with the ordinary physical or chemical forces. It cannot, however, be said that this view has as yet received a scientific demonstration. On the other hand, it seems safer, with our present knowledge, to believe that protoplasm is simply the necessary material basis or vehicle through which vital force is manifested, though we are still unable to speak with any positiveness as to the precise nature of the forces which are the fundamental causes of life.
If, in conclusion, it be asked whether the term "vital force" is any longer permissible in the mouth of a scientific man, the question must, in the meanwhile, be answered in the affirmative. Formerly, no doubt, the progress of science was retarded and its growth checked by a too exclusive reference of natural phenomena to a so-called vital force. Equally unquestionable is the fact that the development of biological science has progressed contemporaneously with the successive victories gained by the physicists over the vitalists. Still, no physicist has hitherto succeeded in explaining any fundamental vital phenomenon upon purely physical and chemical principles. The simplest vital phenomenon has in it something over and above the merely chemical and physical forces which we can demonstrate in the laboratory. It is easy, for example, to say that the action of the gastric juice is a chemical one, and doubtless the discovery of this fact was a great step in physiological science. Nevertheless, in spite of the most searching investigations, it is certain that digestion presents phenomena which are as yet inexplicable upon any chemical theory. This is exemplified in its most striking form, when we look at a simple organism like the Amoeba. This animalcule, which is structurally little more than a mobile lump of semi-fluid protoplasm, digests as perfectly - as far as the result to itself is concerned - as does the most highly organised animal with the most complex digestive apparatus. It takes food into its interior, it digests it without the presence of a single organ for the purpose; and, still more, it possesses that inexplicable selective power by which it assimilates out of its food such constituents as it needs, whilst it rejects the remainder. In the present state of our knowledge, therefore, we must conclude that even in the process of digestion, as exhibited in the Amoeba, there is something that is not merely physical or chemical. Similarly, any organism when just dead, consists of the same protoplasm as before, in the same forms, and with the same arrangement; but it has most unquestionably lost a something by which all its properties and actions were modified, and some of them were produced. What that something is, we do not know, and perhaps never shall know; and it is possible, though highly improbable, that future discoveries may demonstrate that it is merely a subtle modification of some physical force. In the meantime, as all vital actions exhibit this mysterious something, it would appear unphilosophical to ignore its existence altogether, and the term "vital force" may therefore be retained with advantage. In using this term, however, it must not be forgotten that we are simply employing a convenient expression for an unknown quantity, for that residual portion of every vital action which cannot at present be referred to the operation of any known physical force.
It must, however, also be borne in mind that this residuum is probably not to be ascribed to our ignorance, but that it has a real existence. It appears, namely, in the highest degree probable that every vital action has in it something which is not merely physical and chemical, but which is conditioned by an unknown force, higher in its nature and distinct in kind as compared with all other forces. The presence of this "vital force" may be recognised even in the simplest phenomena of nutrition; and no attempt even has hitherto been made to explain the phenomena of reproduction by the working of any known physical or chemical force.