PROTECTIVE RESEMBLANCES IN VERTEBRATA 65 



roundings is explicable by the operation of natural 

 selection, while the gradual loss of colour when natural 

 selection ceases to operate, is in opposition to Mr. 

 Lucas's hypothesis, which assumes that the colour of 

 the shell is determined by the influence of surround- 

 ing tints upon the retina of the female. If the rusty 

 spots on the eggs of birds of prey are due, as Mr. 

 Lucas supposes, to the sight of blood, eggs laid in the 

 dark should still be affected by the memory of some 

 colour either predominant in the surroundings, or of 

 especial interest to the female. The hypothesis might 

 easily have been tested by surrounding birds with un- 

 usual colours and observing the tints of their eggs : 

 and, had this been done, I believe that the paper would 

 not have been written. 



Since the last paragraph was printed, a letter from 

 the Rev. F. F. Grensted has been communicated to 

 ' Nature ' by Mr. A. E. Wallace. 1 The writer believes 

 that the colour of the egg of the red-backed shrike 

 varies with the tint of the lining material of the nest. 

 Mr. E. B. Titchener maintains that there is not suffi- 

 cient evidence for this opinion. At one time Mr. 

 Titchener believed that Variable Protective Resem- 

 blance was exhibited by the eggs of the yellow- 

 hammer and spotted fly- catcher as well as by those 

 of the red-backed shrike. Further observation con- 

 vinced him that the evidence was insufficient. 2 



1 Nature, vol. 41, Nov. 21, 1889, p. 53. 



2 Ibid., Dec. 12, 1889, pp. 129-30. 



