6i6 LIFE : OUTLINES OF GENERAL BIOLOGY 



and removing adherent particles. There is also the fact that the silk 

 thread that issues from the larva's mouth is sometimes very sticky, 

 and readily adheres to other threads and to adjacent fragments. 

 Then there is the custom of moving the cocoons about from place 

 to place when there is danger or need of sunshine. In some such way 

 the "tailoring " may have arisen. But the beginning of this story may 

 probably remain a perhaps. 



LINKED INSTINCTS. The effectiveness of instinctive behaviour 

 depends (i) on the fact that the efficiency is part of the hereditary 

 equipment, not requiring to be learned; (2) on the automatic 

 readiness with which the response comes to a stimulus, without 

 hesitancy or fumbling; and (3) on the linking of one group of reflex 

 activities to another, so that the completion of A serves as the 

 stimulus to the commencement of B, and so onwards. This idea of 

 "chain-instincts", as they have been called — perhaps linked instincts 

 is clearer — deserves illustration. 



In birds, for instance, the instinctive activities connected with 

 reproduction follow one another in a definite harmonious series, 

 modifiable, however, by intelligence to a degree unknown among 

 insects. As Herrick suggests in his luminous essay on the cuckoo, the 

 instinctive bird-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 series of related impulses. Perhaps it is enough 

 in the first instance to think more objectively of each circle as a group 

 of reflex activities. They are conditioned, no doubt, by externsil 

 influences and internal impulses, but these are in most cases very 

 imperfectly known. We are not at present raising the question of the 

 degree of awareness and endeavour behind the behaviour. 



The cycle varies for different birds, but the commonest sequence, 

 adopted by Merrick, is: 



1. Migration (often followed by choice of "territory"); 



2. Mating (with often complex courtship); 



3. Nest-building; 



4. Egg-laying in the nest ; 



5. Incubation and care of eggs; 



6. Care of young in the nest ; 



7. Care and education of the 3'oung out of the nest ; 



8. vScattering, and migration anew. 



A group of reflex activities may include many often-repeated 

 linkages of reflex acts (physiologically considered), as in nest- 

 building; or it may be very monotonous, as in close incubation; 

 and each continues, with what we may metaphorically call "momen- 

 tum", until there is a change in external influence or in internal 

 impulse. Thus the hatching out of the young birds is a liberating 



