44 THE ECOLOGY OF ANIMALS 



full range of physical conditions possible for it. The 

 method is of practical value in predicting either the 

 future spread of an introduced pest, or the possible 

 spread in favourable years when chmatic limits have 

 themselves expanded. Nichols (1933) worked out 

 cHmographs for pure-bred flocks of British sheep 

 and applied them tentatively to the forecasting 

 of the regions in Australia and New Zealand 

 most suitable for the estabhshment of these 

 breeds. Climographs are a very convenient technique 

 in the study of many problems, and they are 

 fully discussed by Shelf ord (1929) and Chapman 

 (1931). 



In all this discussion it has been assumed that 

 every animal is living in a certain habitat, and that 

 phj^sical and chemical conditions (as well as vegeta- 

 tion and associated animals) are those in which the 

 animal can exist and breed successfully. But how 

 does an animal find its habitat ? We enter here on 

 a most intricate and interesting subject, which 

 depends partly on a proper comprehension of the 

 principles of animal psychology. It raises the whole 

 question of the nature of animal reactions in general. 

 Without going into psychological theories it may be 

 said that there are four important questions which 

 concern the animal ecologist. The first question is : 

 Do animals have definite reactions which enable 

 them to find the habitat suitable to their physiological 

 and ecological characteristics ? Some animals are 

 dispersed by broadcasting methods and arrive in 

 large numbers in habitats both suitable and unsuit- 

 able to them. If the}^ come to the right place they 

 survive, if to the wrong place they die. Examples 

 are young spiders spread by gossamer, and the 

 floating larvae of many marine animals. Davis 

 (1923) has shown that local variations in numbers 

 of the mollusc {Spisula subtruncata) on the Dogger 

 Bank are probably due sometimes to the spat falling 

 on an unsuitable substratum. There remains, how- 



