In another respect these microscopic organisms are of vital importance. Many diseases are now known, and others suspected, to be entirely due to Bacteria and other minute forms of life (Microbes), which multiply incredibly, and either destroy their victims, or after a while diminish again in numbers. We live indeed in a cloud of Bacteria. At the observatory of Montsouris at Paris it has been calculated that there are about 80 in each cubic meter of air. Elsewhere, however, they are much more numerous. Pasteur's researches on the Silkworm disease led him to the discovery of Bacterium anthracis, the cause of splenic fever. Microbes are present in persons suffering from cholera, typhus, whooping-cough, measles, hydrophobia, etc., but as to their history and connection with disease we have yet much to learn. It is fortunate, indeed, that they do not all attack us.
In surgical cases, again, the danger of compound fractures and mortification of wounds has been found to be mainly due to the presence of microscopic organisms; and Lister, by his antiseptic treatment which destroys these germs or prevents their access, has greatly diminished the danger of operations, and the sufferings of recovery.
SIZE OF ANIMALS
In the size of animals we find every gradation from these atoms which even in the most powerful microscopes appear as mere points, up to the gigantic reptiles of past ages and the Whales of our present ocean. The horned Ray or Skate is 25 feet in length, by 30 in width. The Cuttle-fishes of our seas, though so hideous as to resemble a bad dream, are too small to be formidable; but off the Newfoundland coast is a species with arms sometimes 30 feet long, so as to be 60 feet from tip to tip. The body, however, is small in proportion. The Giraffe attains a height of over 20 feet; the Elephant, though not so tall, is more bulky; the Crocodile reaches a length of over 20 feet, the Python of 60 feet, the extinct Titanosaurus of the American Jurassic beds, the largest land animal yet known to us, 100 feet in length and 30 in height; the Whalebone Whale over 70 feet, Sibbald's Whale is said to have reached 80-90, which is perhaps the limit. Captain Scoresby indeed mentions a Rorqual no less than 120 feet in length, but this is probably too great an estimate.
COMPLEXITY OF ANIMAL STRUCTURE
The complexity of animal structure is even more marvellous than their mere magnitude. A Caterpillar contains more than 2000 muscles. In our own body are some 2,000,000 perspiration glands, communicating with the surface by ducts having a total length of some 10 miles; while that of the arteries, veins, and capillaries must be very great; the blood contains millions of millions of corpuscles, each no doubt a complex structure in itself; the rods in the retina, which are supposed to be the ultimate recipient of light, are estimated at 30,000,000; and Meinert has calculated that the gray matter of the brain is built up of at least 600,000,000 cells. No verbal description, however, can do justice to the marvellous complexity of animal structure, which the microscope alone, and even that but faintly, can enable us to realise.
LENGTH OF LIFE
How little we yet know of the life-history of Animals is illustrated by the vagueness of our information as to the age to which they live. Professor Lankester[18] tells us that "the paucity and uncertainty of observations on this class of facts is extreme." The Rabbit is said to reach 10 years, the Dog and Sheep 10-12, the Pig 20, the Horse 30, the Camel 100, the Elephant 200, the Greenland Whale 400 (?): among Birds, the Parrot to attain 100 years, the Raven even more. The Atur Parrot mentioned by Humboldt, talked, but could not be understood, because it spoke in the language of an extinct Indian tribe. It is supposed from their rate of growth that among Fish the Carp is said to reach 150 years; and a Pike, 19 feet long, and weighing 350 lbs., is said to have been taken in Suabia in 1497 carrying a ring, on which was inscribed, "I am the fish which was first of all put into the lake by the hands of the Governor of the Universe, Frederick the Second, the 5th Oct. 1230." This would imply an age of over 267 years. Many Reptiles are no doubt very long-lived. A Tortoise is said to have reached 500 years. As regards the lower animals, the greatest age on record is that of Sir J. Dalzell's Sea Anemone, which lived for over 50 years. Insects are generally short-lived; the Queen Bee, however, is said by Aristotle, whose statement has not been confirmed by recent writers, to live 7 years. I myself had a Queen Ant which attained the age of 15 years.
The May Fly (Ephemera) is celebrated as living only for a day, and has given its name to all things short-lived. The statement usually made is, indeed, very misleading, for in its larval condition the Ephemera lives for weeks. Many writers have expressed surprise that in the perfect state its life should be so short. It is, however, so defenceless, and, moreover, so much appreciated by birds and fish, that unless they laid their eggs very rapidly none would perhaps survive to continue the species.
Many of these estimates are, as will be seen, very vague and doubtful, so that we must still admit with Bacon that, "touching the length and shortness of life in living creatures, the information which may be had is but slender, observation is negligent, and tradition fabulous. In tame creatures their degenerate life corrupteth them, in wild creatures their exposing to all weathers often intercepteth them."