Page:The British Warblers A History with Problems of Their Lives - 9 of 9.djvu/50

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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 accumulation, or by the sudden appearance of a physiological development in the young bird, plus the sudden appearance of the corresponding instinct in the parent? In a case of this description Professor Lloyd Morgan's theory^[1] 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 combination 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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[1] 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 combination 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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