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.