In like manner the gap supposed to exist between the kingdoms of the non-living and living is closing up. As long as men had only studied the higher forms of living things there was no difficulty in defining and distinguishing living organisms. To define and to distinguish the lowest forms of those now known is impossible. How completely this is true can only be understood by those who have studied the protoplasmic masses that hover on the border line between the organic and the inorganic. But even the unskilled in microscopic work will be able to grasp something of the great truth if they will take the trouble to look up the innumerable definitions of life that have been given by various persons, and note how unsatisfactory, how contradictory and often self-contradictory they are.
If we pass up into the kingdoms of the living, and study plants and animals, the same unity of phenomena meets us. Our classification terms – order, genus, species, and so forth – are as artificial as our names for the geological systems. No one holds to-day that any single species is clearly marked off from all others. Connecting links abound in our vegetable kingdom. The lichens, long regarded as a separate class of lowly organised plants are now known to be fungi that are parasitic upon algae. The higher cryptogams or flowerless plants are found to be at one in their structure and functions with the lower phsenogams or flowering plants.
The distinctions between plants and animals are found to have vanished. Once again it is easy enough to distinguish high plants from high animals. But no man can satisfactorily draw the line between the lower members of the two kingdoms. The old definitions of the animal and the plant given with a suicidal glibness in old books on botany and zoology, when tried in the balance of criticism, are found wanting. Even the food-distinction, supposed to be the best distinction between the two groups, fails. It is no longer true that plants feed on the inorganic, and animals on organic substances. The cases of vegetable parasites and of insectivorous plants give a direct contradiction to this statement. And it is very interesting to notice how gradual are the transitions in this as in all cases. A group of plants known as saprophytes, that feed on decaying organic things, is the natural transition between the ordinary plants that eat inorganic food-stuffs, and those plants that, like animals, exist on organic substances. So marked is this difficulty of distinguishing between the lower plants and the lower animals, that it has been suggested that a third kingdom of the living should be constructed midway between the two generally recognised. This is to be called Protista, and is to include all the doubtful forms that are not clearly members either of the Kingdom Animalia or of the Kingdom Vegetabilia.
If the arbitrary nature of all our systems of classification is understood, this new division will do little harm. But for the systematist the difficulty is by the establishment of this group only doubled. Heretofore he had only to struggle over a particular living, thing, with the view to determine whether it were plant or animal. Now he will have to struggle over it with the view of telling whether it is Protistic or animal, or Protistic or vegetable. But the true evolutionist will only look on the group of the Protista as containing forms that represent the parent condition of both vegetables and animals.
The animal kingdom, no less than the vegetable, gives these results. Amphioxus, the little Mediterranean fish, links the Vertebrata, or back-boned animals, for ever on to the Invertebrata. The classes of the Vertebrate sub-kingdom have their connecting links or intermediate forms. These classes, adopting for popular exposition the old classification, are the Pisces, Amphibia, Eeptilia, Aves, Mammalia. Whilst Amphioxus at the lower end of the class of fishes connects these with the soft-bodied animals, or Mollusca, at the upper end of the Pisces, we have the Lepidosiren, or mud-fish. It is impossible to say whether this animal is more of a fish or a reptile. With limbs rather than fins, with three cavities to its heart, and a swim-bladder that acts as a lung, it has yet so many parts of its anatomy that are piscine as to lead Professor Huxley still to place it as a solitary representative of the highest order of Pisces.
The class Amphibia is itself a confirmation of the general truth, for its members, such as the frogs, are in their early condition fish, and in their adult state reptiles. Pterodactyl of the Jurassic strata is the winged lizard. Its name tells us that we have a form intermediate between the classes Reptilia and Aves. The duck-billed Platypus, or Ornithorhyncus, of Australia, is a furred mammal that suckles its young, and yet has a bird's bill, a bird's feet, a bird's wishing-bone, a bird's heart, a bird's alimentary canal. If we turn to the individual classes, the same thing obtains. To take but the the highest class, the Prosimiae, or half-apes, among the Mammalia are an order, that stands centrally to the Insectivora, Eodentia, Cheiroptera, and Primates. There is no gap between man and the rest of the Primates. Not a single mark of anatomy, of physiology, or of psychology, clearly distinguishes man from the highest apes.
If we study the individual animal, the same fact of the unity of phenomena is again borne in upon us. The bodily functions are by no means so distinct in their nature as we were wont to think. To take but an illustration.
The sense-organs of man are all found to be only so many modifications of the integument.
The skin or tactile organ is the integument. The tongue or taste organ is but the integument folded inwards and a little modified. The nasal cavities are also lined with a modification of the same tissue, and even the most complex sense organs that are at the same time the most important – that is the eye and the ear – are, as the study of development or embryology shows us, only the result of a series of remarkable changes affecting certain parts of the epidermis of the animal.