In the past few decades, remarkable findings have been made in ethology, the study of animal social behavior. Earlier scientists had【21】that nonhuman social life was almost totally instinctive or fixed by genetics. Much more careful observation has shown that【22】variation occurs among the social ties of most species, showing that [earning is a part of social life. That is, the【23】are not solely fixed by the genes.【24】, the learning that occurs is often at an early age in a process that is called imprinting. Imprinting is clearly【25】instinctive, but it is not quite like the learning of humans, it is something in between the two. An illustration best【26】the nature of imprinting. Once, biologists thought that ducklings followed the mother duck because of instincts. Now we know that. shortly【27】they hatch, ducklings fix【28】any object about the size of a duck and will henceforth follow it. So ducklings may follow a basketball or a briefcase if these are【29】 for the mother duck at the time when imprinting occurs. Thus, social ties can be considerably【30】, even ones that have a considerable base【31】by genetics.
Even among the social insects something like imprinting【32】influence social behavior. For example, biologists once thought bees communicated with others purely【33】instinct. But, in examining a "dance" that bees do to indicate the distance and direction of a pollen source, observers found that bees raised in isolation could not communicate effectively. At a higher level, the genetic base seems to be much more for an all-purpose learning rather than the more specific responses of imprinting. Chimpanzees, for instance, generally【34】very good mother but Jane Goodall reports that some chimps carry the infant. upside down or【35】fail to nurture the young.
(21)

A. assumed
B. adopted
C. believed
D. surmised