The first three chapters describe the basic requirements for flight, which are essentially the same for all flying animals and also for aircraft. They all need lift, to stop them falling to the ground. Lift is generated when the wings move forward through the air, and this requires the application of force, which is called thrust. In animals the thrust is provided by flapping. So powered flight is flapping flight.
Flapping is not a simple up-and-down movement. On the downstroke the wing is tilted so that the lift points slightly forwards as well as upwards; this is where the thrust comes from. (I have not found this explained so clearly anywhere previously.) Most of the useful forces, lift and thrust, are produced during the downstroke. The upstroke is mainly needed to bring the wing into position for the next downstroke, although some fliers—mainly insects—can produce a useful amount of thrust in the upstroke.
Aerodynamic efficiency is clearly essential for flight but is not the only requirement. Another is good vision, and so, perhaps less obviously, is a sophisticated nervous system to control flying and landing. In the case of bats a new sense, echolocation, developed, because most bats are nocturnal. But Alexander makes the important point that, contrary to the popular expression 'blind as a bat', bats actually have very good vision.
Gliding does not require the production of power by the glider. It uses gravity to move the wings through the air and generate lift and is really a form of controlled falling; Alexander explains this with the nice analogy of a bicyclist freewheeling downhill.
The question of size is very important in relation to flight. The smallest flying creatures, thrips, are half a millimetre long, while the wandering albatross has a wingspan of almost 3.5 metres and one pterosaur was the size of a small aircraft. Flight means quite different things for the very small and the very big. At the smallest scale the viscosity of the air becomes important and this explains why very small insects may have wings that are more like bristly rods or loose feathers than what we usually think of as insect wings.
Chapter 4 compares gliding with flapping and looks at a gliding lizard and some extinct lizard-like gliders. The remaining chapters are mainly concerned with flapping flight. Each of the four groups of fliers has a chapter to itself, reviewing what we know, or don't know, about how it evolved. Evidence from fossils is clearly important here but in recent decades this has been supplemented by genetic studies, at least in the case of birds, bats, and insects, since these groups have living representatives; it is not possible to extract DNA from pterosaur fossils.
The fossil evidence for birds is quite good: not only do we have the famous Archaeopteryx, but recent discoveries in China have provided a rich store of information about the presence of feathers in many dinosaurs. This has largely settled the question of whether birds descended from dinosaurs; they did. For the other three groups the situation is more frustrating. We do not have fossils showing the intermediate stages of development for pterosaurs or bats, and although there are very early insect fossils they show them with wings already fully evolved and functional.
There is a long-standing and sometimes quite vigorous debate among specialists about how powered (flapping) flight originated. Did it arise in animals that ran along the ground and occasionally leapt into the air or in gliders who jumped or fell out of trees? Ingenious arguments have been made to support both of these ideas in the case of insects and birds (an 'arboreal' origin of flight is widely accepted for pterosaurs and bats).
Alexander discusses the question at some length and concludes that the 'above down' view is more likely to be correct. Many insects that lack wings, such as worker ants, are able to control the direction of falls to some extent, and this can be seen as an elementary form of gliding that could develop into flapping flight. It is more difficult to decide how insect wings evolved; one possibility is by adaptation ('exaptation') of gills, the other is outgrowth from the thorax. In the absence of helpful fossils it is impossible to decide between these hypotheses.
In the case of birds the 'ground up' idea was given a new twist when it was noted that young birds with still undeveloped wings could run up slopes faster by flapping their wings. This seemed to suggest a means by which small feathered dinosaurs might have moved towards flapping flight. But Alexander point out that the only animals that show this behaviour today are those that have already acquired flight, so they already have the anatomical and nervous system adaptations that flight requires.
Although flying has a number of obvious advantages it has important costs, including a reduction in the ability to walk easily on the ground, at least for bats and pterosaurs; take-off has considerable energy requirements in getting airborne in the case of heavier fliers. A number of bird species have lost flight for various reasons: some lived on islands where there were no predators, for example, and others, such as penguins, have adapted their wings for 'flying' underwater. Very large birds such as ostriches and emus are too heavy to fly. Some insects, such as fleas and lice, which evolved from flying ancestors have also lost their wings. What is perhaps surprising is that evolution never seems to take place in the reverse direction; flight once lost is never recovered.
Alexander tells us that this book had a long gestation; he started writing it in 2002. But if the writing was difficult the reading is easy; the book ranks highly on that score. But Alexander has not compromised on the scientific content, and he gives all the different views he describes a fair hearing while not hesitating to say where his own preference lies. This is definitely a book that I shall be reading more than once.