The Outline of Science, Volume 1

There are three chief kinds of death, (a) The great majority of animals come to a violent end, being devoured by others or killed by sudden and extreme changes in their surroundings. (b) When an animal enters a new habitat, or comes into new associations with other organisms, it may be invaded by a microbe or by some larger parasite to which it is unaccustomed and to which it can offer no resistance. With many parasites a "live-and-let-live" compromise is arrived at, but new parasites are apt to be fatal, as man knows to his cost when he is bitten by a tse-tse fly which infects him with the microscopic animal (a Trypanosome) that causes Sleeping Sickness. In many animals the parasites are not troublesome as long as the host is vigorous,[Pg 72] but if the host is out of condition the parasites may get the upper hand, as in the so-called "grouse disease," and become fatal. (c) But besides violent death and microbic (or parasitic) death, there is natural death. This is in great part to be regarded as the price paid for a body. A body worth having implies complexity or division of labour, and this implies certain internal furnishings of a more or less stable kind in which the effects of wear and tear are apt to accumulate. It is not the living matter itself that grows old so much as the framework in which it works—the furnishings of the vital laboratory. There are various processes of rejuvenescence, e.g. rest, repair, change, reorganisation, which work against the inevitable processes of senescence, but sooner or later the victory is with ageing. Another deep reason for natural death is to be found in the physiological expensiveness of reproduction, for many animals, from worms to eels, illustrate natural death as the nemesis of starting new lives. Now it is a very striking fact that to a large degree the simplest animals or Protozoa are exempt from natural death. They are so relatively simple that they can continually recuperate by rest and repair; they do not accumulate any bad debts. Moreover, their modes of multiplying, by dividing into two or many units, are very inexpensive physiologically. It seems that in some measure this bodily immortality of the Protozoa is shared by some simple many-celled animals like the freshwater Hydra and Planarian worms. Here is an interesting chapter in evolution, the evolution of means of evading or staving off natural death. Thus there is the well-known case of the Paloloworm of the coral-reefs where the body breaks up in liberating the germ-cells, but the head-end remains fixed in a crevice of the coral, and buds out a new body at leisure. Along with the evolution of the ways of avoiding death should be considered also the gradual establishment of the length of life best suited to the welfare of the species, and the punctuation of the life-history to suit various conditions. Photo: J. J. Ward, F.E.S. GREEN HYDRA A little freshwater polyp, about half an inch long, with a crown of tentacles round the mouth. It is seen giving off a bud, a clear illustration of asexual reproduction. When a tentacle touches some small organism the latter is paralysed and drawn into the mouth. Photo: J. J. Ward, F.E.S. EARTHWORM Earthworms began the profitable habit of moving with one end of the body always in front, and from worms to man the great majority of animals have bilateral symmetry. DIAGRAM ILLUSTRATING THE BEGINNING OF INDIVIDUAL LIFE 1. An immature sperm-cell, with 4 chromosomes (nuclear bodies) represented as rods. 2. A mature sperm-cell, with 2 chromosomes. 3. An immature egg-cell, with 4 chromosomes represented as curved bodies. 4. A mature egg-cell, with 2 chromosomes. 5. The spermatozoon fertilises the ovum, introducing 2 chromosomes. 6. The fertilised ovum, with 4 chromosomes, 2 of paternal origin and 2 of maternal origin. 7. The chromosomes lie at the equator, and each is split longitudinally. The centrosome introduced by the spermatozoon has divided into two centrosomes, one at each pole of the nucleus. These play an important part in the division or segmentation of the egg. 8. The fertilised egg has divided into two cells. Each cell has 2 paternal and 2 maternal chromosomes. Reproduced from the Smithsonian Report, 1917. GLASS MODEL OF A SEA-ANEMONE A long tubular sea-anemone, with a fine crown of tentacles around the mouth. The suggestion of a flower is very obvious. By means of stinging lassoes on the tentacles minute animals on which it feeds are paralysed and captured for food. [Pg 73] THIS DRAWING SHOWS THE EVOLUTION OF THE BRAIN FROM FISH TO MAN The Cerebrum, the seat of intelligence, increases in proportion to the other parts. In mammals it becomes more and more convoluted. The brain, which lies in one plane in fishes, becomes gradually curved on itself. In birds it is more curved than the drawing shows.