THE WEB OF LIFE 315 



mantle, we may distinguish (1) those formed around an 

 extrinsic soHd inorganic nucleus such as a quartz particle ; 

 (2) those formed around an extrinsic organic nucleus, such 

 as a parasite, an ovum, or a fragment of tissue ; (3) those 

 formed around a minute centre of the shell-forming 

 organic substance, called conchin. Very interesting ex- 

 periments have been made by Alverdes, who introduced 

 fragments of tissue into the mantle or skin of mussels 

 and found that they were surrounded by concentric layers 

 of mother-of-pearl. 



Domestic Complications 



We have already seen that in some cases an animal 

 cannot continue its kind without the unconscious assistance 

 — ^we can hardly say co-operation — of other creatures. 

 The mussel needs the minnow and the bitterling needs 

 the mussel. But it seems almost necessary to separate 

 off from such cases, where, after all, parental care is quite 

 in evidence, such remarkable occurrences as the cuckoo's 

 habit of handing over the responsibilities of nurture to 

 a foster-parent. There is no more extraordinary story 

 in the whole range of Natural History, and it becomes 

 more wonderful the more we probe into its details. 



Habits of the Cuckoo. — As Aristotle knew so many 

 years ago, the European Cuckoo {Cuculus canorus), foists 

 her several eggs, at intervals of a few days, into the 

 nests of various more or less appropriate birds. These 

 foster-parents, unconscious of being fooled, or indifferent 

 to such considerations, hatch the cuckoo's egg among 

 their own and feed the hungry seK-assertive nestling 

 at the expense of no small wear and tear, and at the 



