i62 THEORETICAL BIOLOGY 



animals. In parasites we find prehensile feet which are 

 exactly inter-adjusted with the tissues of the hosts furnishing 

 them with a medium. 



There can be as little question about inter-adjustment in 

 the sex-circle. The mutual differentiations of the sexual 

 organs in both male and female, extending even to the finest 

 detail, is developed in butterflies to an especially instructive 

 degree. The study of the secondary sexual organs yields us 

 an inexhaustible supply of information regarding an inter- 

 adjustment that is positively marvellous. For here it is not 

 merely a question of an anatomical interlocking of effectors 

 constructed in pairs, but of an inter-adjustment which 

 connects the effectors of one sex with the receptors of the 

 other. There are female butterflies producing a substance 

 not yet demonstrable by chemical or physical means, the 

 existence of which Fabre could prove only by the fact that 

 the place where the female had settled attracted the males 

 towards it from miles around. 



All conceivable effects, optic, acoustic and tactile, are 

 employed in the sexual life of animals in order to bring about 

 so-called sexual selection. That is to say, there is everywhere 

 an extraordinarily delicate inter-adjustment, which secures, 

 even if only for a short period, the association of the two sexes. 



Another function-circle, which we have not yet mentioned, 

 connects parents and children among the higher animals. 

 We have only to remind ourselves of the familiar instance of 

 parental care, and we are at once convinced that here also 

 there is inter-adjustment. 



We shall take only one example of this, which happens to 

 be especially instructive. From studying Boldamus' vast 

 collection of eggs, I have convinced myself that the female 

 cuckoo always lays in the nest of other birds eggs that re- 

 semble theirs in colour. Which shows that the effector organs 

 of the cuckoo are harmonised with the mark-organs of other 



