MORLEY ROBERTS 165 



the very outset we are confronted with the upsetting 

 fact of many species that go further north to breed — 

 in latitude of 80° to 85° — and fly south when the 

 Arctic summer is over, but do not come to a stop 

 when they have entered into warm regions, but fly 

 on and on and across the zone of greatest heat and 

 still on until they have left the Equator 30° to 40° 

 or 45° behind, and would probably go further towards 

 the Antarctic if the conditions permitted. 



I think it was Aristotle who once remarked that it 

 is always best to get the facts and then consider the 

 causes. Doubtless he meant all the facts. 



To conclude the survey, I will quote from a letter 

 written to me on these questions by my friend 

 Morley Roberts, whose recent work on Warfare 

 in the Human Body entitles him to a respectful 

 hearing. 



He begins hopefully: " The problem of bird migra- 

 tion, though one of the most difficult in zoological 

 investigation, does not seem to me wholly incapable 

 of solution." His notion is what I have called the 

 sun-theory, and he has been led to it by a study of the 

 movements or reactions of the minute marine plant- 

 animal, Convoluta roscoffensis. The north and south 

 movements of birds " suggests a theory of negative 

 and positive tropisms, a theory of behaviour, en- 

 forced by light and heat, which has become part of 

 the avian nervous and muscular mechanism. This 

 would mean the acquisition of a north and south 

 ' sense ' (or set of reactions), which would save those 

 who went towards or from the sun." He then 



