Page:The British Warblers A History with Problems of Their Lives - 9 of 9.djvu/53

 congenital factor. What then is the meaning of the evidence derived from birds in captivity? An answer to this question may perhaps be found in the power of imitation so highly developed in many species. This remarkable power is referred to in the lives of the Blackcap and Reed Warbler, more fully in that of the Marsh Warbler, and some idea of the nature and extent of the development can be gained by studying the lists of voices and songs imitated. It is apparently so simple a matter for certain species to reproduce alien strains that one may well ask how a definite type could have been evolved in the absence of any instinctive basis. I should not demur to song being spoken of as a matter of imitation, though founded on an instinctive basis, if by this is meant that a stimulus of an appropriate kind is necessary to set it agoing, but we should not be then justified in speaking of song as having been learnt by imitation. The power of imitation and the true song must be two separate manifestations of one process; may not the absence of the true song in the case of birds reared in captivity be due to an absence of the appropriate stimulus, just as in the absence of appropriate stimuli other hereditary co-ordinations refuse to respond, the alien cries being the stimulus to the power of imitation which must also be founded on a congenital basis.

The more one reflects upon this problem of vocal imitation the more difficult it becomes to estimate its true value from the biological standpoint. Dr. Thorndike regards the phenomena presented by imitative birds as a specialization removed from the general course of mental development. Others think that it has a special part to play in the sexual process, and while dissenting from this view—my reasons for doing so will be found in the discussion of the subject in the life of the Marsh Warbler—I should be the first to admit the incompleteness of our knowledge in regard to the whole matter.

The variation in different districts is referred to in the history of the Lesser Whitethroat, and the suggestion is there