In the past few decades, remarkable findings have been made in ethnology, the study of animal social behavior. Earlier scientists had 1 that nonhuman social life was almost totally instinctive or fixed by genetics. Much more careful observation has shown that 2 variations occur among the social ties of most species, showing that learning is a part of social life. That is, the 3 are not solely fixed by the genes.

　　4，the learning that occurs is often at an early age in a process that is called imprinting. Imprinting is clearly 5 instinctive, but it is not quite like the learning of humans; it is something in between the two. An illustration best 6 the nature of imprinting. Once, biologists thought that ducklings followed the mother duck because of instincts. Now we know that, shortly 7 they hatch, ducklings fix 8 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 9 for the mother duck at the time when imprinting occurs. Thus, social ties can be considerably 10, even ones that have a considerable base 11 by genetics.

　　Even among the social insects something like imprinting 12 influence social behavior. For example, biologists once thought bees communicated with others purely 13 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 14 very good mother but Jane Good all reports that some chimps carry the infant upside down or 15 fail to nurture the young. She believes that these females were the youngest or the 16 child of a mother. In such circumstances, they did not have the opportunity to observe how their own mother 17 for her young. Certainly adolescent chimps who are still with their mothers when other young are born take much interest in the rearing of their young brother or sister. They have an excellent opportunity to learn, and the social ties that are created between mother and young 18 Good all to describe the social unit as a family. The mother offspring tie is beyond 19; there is some evidence to 20 that ties also continue between siblings of the same sex, that is "brother brother" and "sister sister".

　　1. A assumed B adopted C believed D surmised

　　2. A considerate B considerated C considerable D considering

　　3. A statues B statuses C statutes D statures

　　4. A What’s more B Hence C But D However

　　5. A not B only C but D solely

　　6. A clarifies B classifies C defines D outlines

　　7. A than B before C when D after

　　8. A on B with C in D within

　　9. A appropriated B substituted C assigned D distributed

　　10. A varied B deviated C differed D altered

　　11. A fashioned B modified C influenced D affected

　　12. A may B should C must D can

　　13. A by B out of C from D through

　　14. A prove B make C turn D create

　　15. A otherwise B still C yet D even

　　16. A one B sole C single D only

　　17. A looked B attended C cared D provided

　　18. A guide B cause C direct D lead

　　19. A limitation B imagination C doubt D expectation

　　20. A advise B hint C imply D suggest

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