CHAPTER I 



Introduction 



The gross characteristics of the planet earth are the same for all living 

 things inhabiting it. It has a characteristic gravitational field; it re- 

 ceives radiant energy from outer space to the extent determined by the 

 filtering properties of its atmosphere; it is built of specific kinds of 

 chemical compounds in certain quantitative relationships. The details 

 of these properties differ from place to place on the planet. Some are 

 more conducive to animal life than others, and the uneven distribution 

 of animals in the biosphere is a measure of this and of the adjustments 

 that various forms of life have made to the details. To regulate their 

 relations with the earth's environment, animals have evolved sensing 

 devices for detecting such details as are of direct adaptive value. 



Since we *see' the world with our own particular sensing devices, it 

 is difficult to understand fully how other animals *see' it. What picture 

 of the details of the world is perceived by the bee that sees in the ultra- 

 violet, the snake that is deaf to air-borne sounds but detects infra-red, 

 the shrew that cannot see colour, the electric fish that detects changes 

 in the electric field about it? To appreciate these points of view, let 

 alone understand the manner in which all biological sensing devices 

 work, it is helpful to look first at the world, not at the level of detail 

 perceived by us but at a level that we cannot perceive directly, the 

 elementary particles whose interactions and combinations form 

 the universe. 



Of the thirty or so kinds of particles that make up earth or come to 

 it from outer space, the only stable ones are protons, neutrons, 

 neutrinos, electrons, and photons (Ruderman and Rosenfeld, 1960). 

 Some of these (e.g., photons, neutrinos) occur free; others, usually 

 combined into atoms and molecules. Some, such as electrons and 

 photons with certain energy, have profound effects on living material ; 

 others, such as neutrinos, pass through organisms undetected and 

 without effect. All have different kinds of energy. In the final 

 analysis it is to these particles that animals must make adjustments. 

 Organisms living in other parts of the universe would be exposed to 

 other kinds of particles and would have to evolve differently, each 



in its own milieu. 



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