science?_ob=ArticleURL&_udi=B6VJX-3WV9N0M-6&_coverDate=06%2F01%2F1999&_alid=515930889&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6106&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=50eb60d3e2572dc5349790fc42c0d7ac
science?_ob=ArticleURL&_udi=B6VJX-3WV9N0M-6&_coverDate=06%2F01%2F1999&_alid=515930889&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6106&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=50eb60d3e2572dc5349790fc42c0d7ac