Introduction

Abstract

It is generally agreed that animal life originated in the sea and that adaptive radiation subsequently led to the colonisation of other environments — shores and estuaries, streams and lakes, bog, mountain and desert. In their invasion of these habitats animals left the equable, relatively stable surroundings of the open sea and subjected themselves to the rigours of temperature fluctuations and extremes, a variety of ionic backgrounds, areas of depleted oxygen or the possibility of aerial exposure and potential desiccation. The spur for this radiation presumably lay in the prize of access to unexploited habitats and sources of energy.