It is difficult to explain why some of the old types disappeared. The extinction was never sudden. Formidable competitors may have helped to weed out some; for cuttlefish would tend to exterminate trilobites, and voracious fishes would decimate cuttlefish, just as man himself is rapidly and inexcusably annihilating many kinds of beasts and birds. But, apart from the struggle with competitors, it is likely that some types were insufficiently plastic to save themselves from changes of environment, and it seems likely that others were victims to their own constitutions, becoming too large, or too sluggish, or too calcareous; or, on the other hand, too feverishly active. The "scouts" of evolution would be apt to become martyrs to progress; the "laggards" in the race would tend to become pillars of salt; the path of success was oftenest a via media of compromise. Samuel Butler has some evidence for saying that "the race is not in the long run to the phenomenally swift, nor the battle to the phenomenally strong; but to the good average all-round organism that is alike shy of radical crotchets and old world obstructiveness."

5. Various Difficulties.—Nowadays it seems natural to us to regard the fossils in the rocks as vestiges of a gradual progress or evolution. As some still find difficulty in accepting this interpretation, I shall refer to three difficulties occasionally raised.

(a) It is said that the number of fossils in successive strata does not increase steadily as we ascend to modern times that the numerical strength of the fauna is strangely irregular. Thus (in 1872) it was computed that 10,000 species were known from the early Silurian rocks, while the much later Permian yielded only 300. But those who use such arguments should mention that a large number of the Silurian species were discovered by the marvellous industry of one man in a favourable locality, and that the rocks of the Permian system are ill adapted for the preservation of fossils. Moreover, we cannot compute the relative duration of the different periods, we cannot infer evolutionary progress from the number of species, and we must make many allowances for the imperfections of the record.

(b) It is said that the occurrence of Fishes in the Silurian, and of many highly organised Invertebrates in the still earlier Cambrian, is inconsistent with a theory which would lead us to expect very simple fossil forms to begin with. But to say so is to forget that we have no conception of the vast duration of periods like the Silurian and Cambrian, while the antecedent Archæan rocks in which we might look for traces of simple ancestral organisms have been shattered and altered too thoroughly to reveal any important secrets as to the earliest animals.

(c) It is maintained that organic evolution proceeds very slowly, and that the geologists and biologists demand more millions than the experts in astronomical physics can grant them. But there is considerable difference of opinion as to the unthinkable length of time during which the earth may have been the home of life; we are apt to measure the rate of evolutionary change by the years of a man's lifetime which lasts but for a geological moment; and there is reason to believe that the simpler animals would change and take great steps of progress much more rapidly than those of high degree.

6. Relative Antiquity of Animals,—I have not much satisfaction in submitting the following table showing the relative antiquity of the higher animals. Such a table is only an approximation; it does not suggest the great differences in the duration of the various periods, nor how the classes of animals waxed and waned, nor how some types in these classes dropped off while others persisted. But the general fact which the table shows is true,—in the course of time higher and higher forms of life have come into being. It is true that the remains of mammals are of more ancient date than those of birds, but it is likely that the remains of the earliest birds have still escaped discovery; moreover, the earliest known mammalian remains seem to be of those of very simple types.

CHAPTER XIV

THE SIMPLEST ANIMALS

1. The Simplest Forms of Life-2. Survey of Protozoa-3. The common Amaba-4. Structure of the Protozoa-5. Life of Protozoa -6. Psychical Life of the Protozoa-7. History of the Protozoa 8. Relation to the Earth-9. Relation to other Forms of Life10. Relation to Man

1. The Simplest Forms of Life.—It is likely that the first breath of life was in the water, for there most of the simplest animals and plants have their haunts. Simple they are, as an egg is simple when contrasted with a bird. They are (almost all) unit specks of living matter, each comparable to, but often more complex than, one of the numerous unit elements or cells which compose any higher plant or animal, moss or oak-tree, sponge or man. It is not merely because they are small that we cannot split them into separate parts different from one another,―size has little to do with complexity,but rather because they are unit specks or single cells. But they are not "structureless"; in fact, old Ehrenberg, who described some of them in 1838 as "perfect organisms" and fancied he saw stomachs, vessels, hearts, and other organs within them, was nearer the truth than those who reduce the Protozoa to the level of white of egg. Nor are they omnipresent, swarming in any drop of water. clear water of daily use will generally disappoint, or rather please us by showing little trace of living things. But take a test-tube of water from a stagnant pool, hold it between your eyes and the light, and it is likely that you will see many forms of life. Simple plants and simple animals are there, the former represented by threads, ovals, and spheres in green, the latter by more mobile almost colourless specks or whitish motes which dance in the water. But besides these there are jerky swimmers whose appearance almost suggests their popular name of "water-fleas," and wriggling "worms,"

The

thinner than thread and lither than eels: both of these may be very small, but closer examination shows that they have parts and organs, that they are many-celled not single-celled animals.

Vary the observations by taking water in which hay stems or other parts of dusty dead plants have been steeped for a few days, and even with the unaided eye you will see a thick crowd of the mobile whitish motes which, from their frequent occurrence in such infusions, are usually called Infusorians. Or if a piece of flesh be allowed to rot in an open vessel of water, the fluid becomes cloudy and a thin flaky scum gathers on the surface. If a drop of this turbid liquid be examined with a high power of the microscope, you will see small colourless rods and spheres, quivering together or rapidly moving in almost incalculable numbers. These, though without green colour, are the minutest forms of plant life; they are Bacteria or Bacilli, the practically omnipresent microbes, some of which as disease germs thin our human population, while others as cleansers help to make the earth a habitable dwelling-place.

2. Survey of Protozoa.-Three great types of unicellular animals or Protozoa have been recognised in almost every classification.

(a) The Infusorians, so abundant in stagnant water, have a common character of activity expressed in the possession of actively mobile lashes of living matter known as cilia or flagella. Thus the slipper-animalcule (Paramæcium) is covered with rows of lashing cilia, while smaller, equally common forms, generally known as Monads, are borne along by the undulatory movement of one or two long whips or flagella. The bell-animalcules (Vorticella) which live in crowds,—a white fringe on the water weeds,—are generally fixed by stalks, but are crowned with active cilia at the upper end of the somewhat urn-shaped cell.

(b) In marked contrast to these are the parasitic Protozoa, the Gregarines, which infest most backboneless animals, notably the male reproductive organs of the earthworm or the gut of lobster and cockroach. Sluggishness and the absence of all locomotor processes are their characteristics.

(c) Between these two extremes of activity and passivity there is a third type well represented by the much-talked-of Amaba which glides about on the mud of the pond, by the sun-animalcules (Actinosphærium) which float in the clear water of brooks, by the limy-shelled, chalk-forming Foraminifera which move slowly on seaweeds or at the bottom of shallow water, or in some cases float at the surface of the sea, and by the flinty-shelled Radiolarians which live in the open ocean. In all these the living matter spreads out in thick or thin, stiff or plastic, free or interlacing processes, which often admit of a slow gliding motion, and are still