142 evolution: the ages and tomorrow 



tal feeding reactions. Without these stimuli the bird simply 

 ceases to act like a parent; there does not seem to be any 

 of the grief and distraction that memory produces in some 

 mammalian parents. 



The strangeness and, from an anthropomorphic point of 

 view, undesirability of some of these instincts can probably 

 be best illustrated by bird parasitism, such as that of the 

 cuckoo. The tgg is laid in the nest of another species, often 

 the hedge sparrow, where it hatches in double quick time. 

 The embryonic development has been hurried by evolution 

 so that the young cuckoo v/ill be ahead of the host fledg- 

 lings. As soon as it is hatched, the cuckoo begins to evict 

 everything in the nest, eggs or fledglings, acting automati- 

 cally, not willfully, under the control of an instinct with 

 a thoroughly established sensory and brain pattern. The 

 back of the young cuckoo is slightly hollowed out and 

 highly sensitive. The continued touch of any object there 

 will produce a frantic reaction which drives the cuckoo 

 backward and upward to the edge of the nest where ordi- 

 narily the object is dumped out of the nest. In the mean- 

 time the parent birds offer no objection or give any sign 

 that the tragedy has the least effect upon them. Here is a 

 gaping mouth to feed, and their instinctive, parental com- 

 plex is fully satisfied. 



From the human point of view there is much in instinc- 

 tive behavior we can justly feel is highly undesirable, but 

 we are not justified in condemning all instincts. As we have 

 seen, nature is striving blindly through trial and error, gene 

 and chromosomal variation, guided by natural selection, to 

 explore all possible avenues of evolution. Looking back 

 from a higher level, we feel that there have been too many 

 failures, too many instances of ugly conflicts aided and 

 abetted by the very behavior forces which, if the process 

 is to succeed, will eventually bring life to higher levels of 

 peace and understanding. We can only repeat again that 

 nature seemingly knows no way to drive directly toward 



