In the past few decades, remarkable findings have been made in ethology, 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 obserration has shown that 2 variation occurs 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 fashioned by genetics.
1. A. ossumad 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 来源：www.examda.com 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