THE SEXUAL SELECTION THEORY 139 



remove the faeces of their young from the nest ; thereby 

 preserving it in a sanitary condition. It is certain that 

 any neglect to do this would speedily end in the death of 

 the young. This act is " instinctive " ; it is not per- 

 formed because the parents have evolved any views on 

 sanitation, and any strain in whom this instinct was 

 defective would speedily become eliminated. Mr. Howard 

 has demonstrated the mechanical character of this sanitary 

 measure by placing leaves in nests of young. The parents, 

 having fed their offspring, at once seized upon the leaf 

 and commenced to dispose of it after their usual fashion, 

 first by trying to swallow it and then by carrying it 

 away. They did not, evidently, realize the difference 

 between the texture of the leaf and the milk-white, jelly- 

 like envelope which always encloses the faecal matter of 

 the nestling. We shall probably never know how this 

 most vitally important instinct came into being ; nor can 

 we hope to discover what chain of happenings begot the 

 instinct, which each parent displays, to gently stimulate 

 the cloacal lips of their offspring in order to induce the 

 discharge of the faeces when this does not immediately 

 follow the stimulus of swallowing food. 



We cannot credit these birds with notions on the 

 importance of the regular discharge of the evacuations. 

 Equally mysterious is the development of the envelope 

 enclosing the fsecal matter. This is jelly-like in sub- 

 stance, and of considerable thickness, and is enclosed 

 within a very delicate skin or pellicle, enabling one to 

 lift the whole in the fingers without soiling them. How 

 and where it is formed should not long evade discovery. 

 But how it has come to be is another matter. We can, at 

 any rate, vaguely account for responses of the organism to 

 internal stimuli reacting directly on the individual, but 



