BRITISH WARBLERS 



envelope in the young and the instinct to remove in the 

 parent ? By the slow accumulation of two sets of entirely 

 separate but mutually dependent accidental variations, with 

 nothing but elimination to determine the direction of accumu- 

 lation, or by the sudden appearance of a physiological develop- 

 ment in the young bird, plus the sudden appearance of the 

 corresponding instinct in the parent ? In a case of this des- 

 cription Professor Lloyd Morgan's theory* of the survival of 

 coincident variations is helpful as showing how the acquired 

 modification can determine the survival of congenital variations 

 in the direction of increased accommodation without biological 

 transmission. At first small pieces of the excrement may 

 have been occasionally swallowed for food; but supposing a 

 small amount of intelligence had then been brought to bear 

 upon the situation, leading to removal of part plus the 

 occasional swallowing of part, there would have been a distinct 

 gain to those individuals in whom such modification presented 

 itself. Acquired accommodation would in this case have 



* The theory of the survival of coincident variations is by no means easy 

 to grasp. It may therefore be of some help if I quote, from a private 

 communication, Professor Lloyd Morgan's own statement of his case. He 

 writes thus : " The essential feature of this suggestion may be put as follows : 

 Let M be a modification of instinctive behaviour, such modification being 

 intelligently acquired as the outcome of individual experience ; and let V be 

 a variation of the hereditary tendency to the instinctive behaviour thus 

 susceptible of modification. Now a change of V in the direction of better 

 adaptation, if it have in itself survival value, will be inbred through natural 

 selection. But in itself it may not reach a value which determines survival 

 instead of elimination. Since, however, acquired modifications of behaviour 

 are themselves factors in survival, the organism in which there is a com- 

 bination of favourable variation and favourable modification will stand the 

 best chance. A variation too slight to be" selected, if it stood alone, survives 

 when it is supported by an intelligent modification in a like favourable 

 direction. Hence, though the acquired modification may not itself be directly 

 inherited, it none the less acts as foster-nurse to coincident variation, i.e., 

 those in a like favourable direction. In like manner, where intelligence is 

 lacking, coincident non-adaptive variations will stand the greater chance of 

 elimination. In symbolic terms + M coincident with + V makes for survival ; 

 while — M coincident with - V makes for elimination. The + V. in the 

 survivors is inherited. The - V is bred out of the race." 



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