They have their bitter, nearly microscopic enemies. Upon them, as upon elephants, ride much smaller, bareback riding mites whose acrobatic stunts would be the envy of any circus performer.
“Seven out of 16 ants so examined,” Dr. Weber says, “had mites on them. These mites have no difficulty in moving from one site to another on the ants. A transfer of a mite from one ant to another was watched. It had been riding on one ant when another brushed by waving its antennae over the other as is customary. In a flash the mite grabbed the tip of the left antenna. The ant did not attempt to dislodge the mite although it already had two others on its body. The mite had a rough ride, but was not dislodged.”
The peculiar type of fungus grown by the ant does not grow naturally outside the nest. It can be isolated and cultivated but it quickly is overwhelmed by other fungi in any artificial culture. It is probable that ant and fungi need each other for survival. Possibly the saliva of the insect is essential for the growth of the primitive plant. Likewise the peculiarly developed fungus is essential for the well-being, even for the survival, of the ants. It is one of nature’s partnerships.
Ostracoderms: Ancestors of True Fish
The race of fish first appeared about 350,000,000 million years ago in the Silurian geological era. It was made up of grotesque, clumsy, heavily armored animals who crawled over the ooze of the sea bottoms with very little, if any, capacity to rise or propel themselves in the water. The ascent from such an unpropitious beginning to the swift, graceful swimmers of today is one of the wonder stories of evolution.
These Silurian animals were the ostracoderms. They belonged to the general fish complex but were not in the direct ancestral line of any extant fish. This race continued, in various groupings, for at least 150,000,000 years. The earliest forms were wormlike animals whose fossils are found in ancient rocks of Esthonia. Their heads and the forward parts of their bodies were covered with bony plates. They had no fins to serve for steering and balancing. In appearance they were close to tadpoles. It is quite obvious that they were bottom-dwelling forms who swam, if at all, awkwardly and laboriously. The evolution into more and more efficient swimming animals can be traced through later and later fossils throughout the life history of the race. The body became more flexible. There was a gradual reduction in the thickness of the external armor as the ostracoderms came to depend more and more on speed and less on invulnerability. At the end they probably were comparatively good swimmers.
A little later than the earliest of this long extinct family came the first representatives of the true fish—probably derived from the same general ancestral stock. They also were bottom-dwelling animals, although from the beginning they appear to have been a little better adapted for swimming. In these also, the head and forward part of the body were encased in heavy armor. In ostracoderms, however, this had formed a continuous shell, allowing no anterior freedom of motion in the water. In the earliest true fish it was divided into two parts, the head shield and the body shield. For the most part, however, they could use only the tail and posterior part of the body for propulsion. But through many generations various diversifications of the race became more and more fishlike in form, shed their heavy protective plates, developed paired fins for steering and balance, and continuously improved as swimmers.
“We must take it for granted,” explains Prof. Anatol Heintz, Norwegian paleontologist, “that the ancestral forms of the vertebrates evolved in water. Most primitive forms lived on the bottom and had not yet specialized sufficiently to be able to swim. If the oldest vertebrates were bottom-living or burrowing forms they must have learned to swim, just as later they learned to crawl, walk, run and finally fly.”
Among the earliest groups of true fish were the coelacanths, or “hollow spines.” They left many fossil remains over a period of 200,000,000 years. Supposedly they became extinct about sixty million years ago, at the start of the dawn age when most higher life types known at present first appeared. Through all the vast eons of their existence the “hollow spines” changed little.
Three years ago came one of the outstanding events in present day biology. A living coelacanth was caught by native fishermen off the northeastern coast of Madagascar. It was quite similar to its fossil ancestors—armored head and all. Apparently the Madagascan fishermen had been capturing similar creatures in their nets occasionally for years, without realizing that they were of any particular significance.