THE WEB OF LIFE 321 



or lay in other birds' nest, and when so doing the black- 

 bill has been known to struggle for possession of the stolen 

 nest '. 



Prof. Herrick's general view is that the loss of the nesting 

 instinct in certain cuckoos and cow-birds is due to an 

 irregularity in the rhythm of the reproductive cycle. In 

 most birds the parental instincts or the cyclical instincts 

 connected with reproduction follow one another in a definite 

 harmonious series ' with almost clock-like precision ' 

 though modifiable at every point by intelligence. 



The reproductive cycle is made up of a series of acts 

 or chains of actions, which follow in a definite succession. 

 Eight or more terms may be recognized, but the classifica- 

 tion is unimportant, so long as it is observed that they 

 are serial and harmonious, and that anything which pro- 

 foundly disturbs their normal attunement is disadvan- 

 tageous, and may lead to disaster. If the disturbance 

 is of a fundamental and permanent character, new adjust- 

 ments in the series must follow, if the species survive. 



The cycle may be graphically represented by a number 

 of nearly tangent circles, each of which stands for a distinct 

 sphere of influence or for a subordinate series of related 

 impulses as given in the simplified formula : 



1. Migration ; 



2. Mating ; 



3. Nest-building ; 



4. Egg-laying in nest ; 



5. Incubation and care of eggs ; 



6. Care of young in nest ; 



7. Care and ' education ' of young out of nest ; 



8. Migration. 



One term in the series may be weakened or drop out ; 

 another may be exaggerated and prolonged ; there may 



Y 



