INTRODUCTION 



Xlll 



inhabit thick leaf-litter, or burrow deep into the soil where they 

 are less liable either to become water-logged or desiccated. 



Rigid, mechanistic behaviour patterns in response to environ- 

 mental stimuli have been evolved by means of which the animals 

 find and maintain themselves in suitable habitats. Physiological and 

 morphological adaptations alone would obviously be insufficient 

 to support the life of any free-living animal. Orientation and be- 

 haviour mechanisms must also be evolved to retain organisms in 

 environments to which they are suited, to enable them to find 

 food, mate, and indeed to carry out the innumerable functions es- 

 sential for their continued existence. 



Fig. 2. Woodlice. Trichoniscus pusillus (length 4 mm), Porcellio 



scaber (length 14 mm) and Armadillidium vulgar e (length 1 5 mm). 



(After Webb and Sillem, 1906.) 



The ecology of animals is therefore governed not only by the 

 factors of their environments, physical and biological, but also by 

 their own physiological requirements and behaviour. The inter- 

 relationships between living organisms and their environments 

 include both inter-specific and intra-specific factors. These re- 

 lationships are specific for every organism and continuous through- 

 out its life: they are reciprocal in that the organism is not merely 

 influenced by, but at the same time positively affects its environ- 

 ment, and are indissoluble because the organism cannot exist in- 

 dependently of its environment. Consequently ecology is a vast 

 and complex subject about which comparatively little is yet known. 

 One advantage of this lies in the fact that it is still possible, as 



