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Life originated in the early seas less than a billion years after Earth was formed. Yet another three billion years were to pass before the first plants and animals appeared on the continents. Life's transition from the sea to the land was perhaps as much of an evolutionary challenge as was the genesis of life.

 What forms of life were able to make such a drastic change in lifestyle ? The traditional view of the first terrestrial organisms is based on megafossils — relatively large specimens of essentially whole plants and animals. Vascular plants, related to modern seed plants and ferns, left the first comprehensive megafossil record. Because of this, it has been commonly assumed that the sequence of terrestrialization reflected  the evolution of modern terrestrial ecosystems. In this view, primitive vascular plants first colonized the margins of continental waters, followed by animals that fed on the plants, and lastly by animals that preyed on the plant-eaters. Moreover, the megafossils suggest that terrestrial life appeared and diversified explosively near the boundary between the Silurian and the Devonian periods, a little more than 400 million  years ago.

Recently, however, paleontologists have been taking a closer look at the sediments below this Silurian-Devonian geological boundary. It turns out that some fossils can be extracted from these sediments by putting the rocks in an acid bath. The technique has uncovered new evidence from sediments that were deposited near the shores of the  ancient oceans — plant microfossils and microscopic pieces of small animals. In many instances the specimens are less than one-tenth of a millimeter in diameter. Although they were entombed in the rocks for hundreds of millions of years, many of the fossils consist of the organic remains of the organism.

These newly discovered fossils have not only revealed the existence of previously  unknown organisms, but have also pushed back these dates for the invasion of land by multicellular organisms. Our views about the nature of the early plant and animal communities are now being revised. And with those revisions come new speculations about the first terrestrial life-forms.

The hippopotamus is the third largest land animal, smaller only than the elephant and the rhinoceros. Its name comes from two Greek words which mean "river horse." The long name of this animal is often shortened to the easier to handle term "hippo."

The hippo has a natural affinity for the water. It does not float on top of the water; instead, it can easily walk along the bottom of a body of water. The hippo commonly remains underwater for three to five minutes and has been known to stay under for up to half an hour before coming up for air.

In spite of its name, the hippo has relatively little in common with the horse and instead has a number of interesting similarities in common with the whale. When a hippo comes up after a stay at the bottom of a lake or river, it releases air through a blowhole, just like a whale. In addition, the hippo resembles the whale in that they both have thick layers of blubber for protection and they are almost completely hairless.

Schizophrenia is often confused with multiple personality disorder yet is quite distinct from it. Schizophrenia is one of the more common mental disorders, considerably more common than multiple personality disorder. The term "schizophrenia” is composed of roots which mean "a splitting Line of the mind," but it does not refer to a division into separate and distinct personalities, as occurs in multiple personality disorder. Instead, schizophrenic behavior is generally characterized by illogical thought patterns and withdrawal from reality. Schizophrenics often live in a fantasy world where they hear voices that others cannot hear, often voices of famous people. Schizophrenics tend to withdraw from families and friends and communicate mainly with the “voices” that they hear in their minds.

It is common for the symptoms of schizophrenia to develop during the late teen years or early twenties, but the causes of schizophrenia are not well understood. It is believed that heredity may play a part in the onset of schizophrenia. In addition, abnormal brain chemistry also seems to have a role; certain brain chemicals, called neurotransmitters, have been found to be at abnormal levels in some schizophrenics.

The changing profile of a city in the United States is apparent in the shifting definitions used by the United States Bureau of the Census. In 1870 the census officially distinguished the nation's “urban” from its “rural” population for the first  time. “Urban population” was defined as persons living in towns of 8,000 inhabitants  or more. But after 1900 it meant persons living in incorporated places having 2,500 or more inhabitants.

Then, in 1950 the Census Bureau radically changed its definition of “urban” to take account of the new vagueness of city boundaries. In addition to persons living in incorporated units of 2,500 or more, the census now included those who lived in  unincorporated units of that size, and also all persons living in the densely settled urban fringe, including both incorporated and unincorporated areas located around cities of 50,000 inhabitants or more. Each such unit, conceived as an integrated economic and social unit with a large population nucleus, was named a Standard Metropolitan Statistical Area (SMSA).

 Each SMSA would contain at least one central city with 50,000 inhabitants or more or  two cities having shared boundaries and constituting, for general economic and social purposes, a single community with a combined population of at least 50,000, the smaller of which must have a population of at least 15,000. Such an area included the county in which the central city is located, and adjacent counties that are found to  be metropolitan in character and economically and socially integrated with the county of the central city. By 1970, about two-thirds of the population of the United States was living in these urbanized areas, and of that figure more than half were living outside the central cities.

While the Census Bureau and the United States government used the term SMSA,  by 1969 there were 233 of them, social scientists were also using new terms to describe the elusive, vaguely defined areas reaching out from what used to be simple “towns” and “cities”. A host of terms came into use: “metropolitan regions,” “polynucleated population groups,” “conurbations,” “metropolitan clusters,” “megalopolises,” and so on.

Birds that feed in flocks commonly retire together into roosts. The reasons for roosting communally are not always obvious, but there are some likely benefits.

In winter especially, it is important for birds to keep warm at night and conserve precious food reserves. One way to do this is to find a sheltered roost. Solitary roosters shelter in  dense vegetation or enter a cavity - horned larks dig holes in the ground and ptarmigan burrow into snow banks - but the effect of sheltering is magnified by several birds huddling together in the roosts, as wrens, swifts, brown creepers, bluebirds, and anis do. Body contact reduces the surface area exposed to the cold air, so the birds keep each other warm. Two kinglets huddling together were found to  reduce their heat losses by a quarter and three together saved a third of their heat.

The second possible benefit of communal roosts is that they act as “information centers.” During the day, parties of birds will have spread out to forage over a very large area. When they return in the evening some will have fed well, but others may have found little to eat. Some investigators have observed that when the birds set out  again next morning, those birds that did not feed well on the previous day appear to follow those that did. The behavior of common and lesser kestrels may illustrate different feeding behaviors of similar birds with different roosting habits. The common kestrel hunts vertebrate animals in a small, familiar hunting ground, whereas the very similar lesser kestrel feeds on insects over a large area. The common kestrel roosts and  hunts alone, but the lesser kestrel roosts and hunts in flocks, possibly so one bird can learn from others where to find insect swarms.

Finally, there is safety in numbers at communal roosts since there will always be a few birds awake at any given moment to give the alarm. But this increased protection is partially counteracted by the fact that mass roosts attract predators and are especially  vulnerable if they are on the ground. Even those in trees can be attacked by birds of prey. The birds on the edge are at greatest risk since predators find it easier to catch small birds perching at the margins of the roost.