There are two factors（61）___（determine） an individual's intelligence．The first is the sort of brain he is born（62）___．Human brains differ considerably，some being more capable than others．（63）___however good a brain he has to begin with，an individual will have a low intelligence unless he has（64）___（opportunity） to learn．So the second factor is what happens to the individual-the sort of environment（65）___he is brought up．If an individual is handicapped（受阻碍）environmentally，it is（66）___（like）that his brain will fail to develop and he will never reach the level of intelligence of which he is capable．

The importance of environment in determining an individual's intelligence can be shown by（67）___case of the twins，Peter and Mark．When the twins were three months old，（68）___parents died，and they were placed in separate foster homes．Peter was brought up by parents of low intelligence with poor educational opportunities．Mark was educated in the home of well-to-do parents who（69）___（be） to college．This environmental difference continued until the twins were in their late teens，when they（70）___（give） tests to measure their intelligence．Mark's I．Q．was 125，twenty-five points higher than the average and fully forty points higher than his twin brother．