<br ><br > Since man occupies his unique position in the world mainly as<br >a consequence of his superior brain， the subject of its operation is<br >naturally of great interest. There has been much debate about the<br >exact importance of genetic factors in human intelligence， but peo-<br >ple have always felt that inherited differences play a role， some per-<br >sons being born with better mental endowment than others. Recent<br >concern about declining scores on scholastic aptitude tests， ad-<br >ministered to prospective college students， also make timely a re-<br >consideration of the evidence for heredity in human intellectual de-<br >velopment.<br > Human genetics， in actual practice， has for a long time dealt<br >primarily with abnormalities or diseases， the tools for an approach<br >to a study of normal or superior development being， therefore， rela-<br >tively undeveloped. The basic concepts are， however， the same， de-<br > pendent on the transmission of individual pairs of genes， each<br > chromosomal locus being responsible for a particular biochemical<br > process. While abnormalities are usually easy to recognize， the nor-<br > mal counterparts are frequently less visible. Diseases can thus often<br > be identified with particular genes， whereas studies of normal char-<br > acteristics and their range of variation remain less specific in na-<br > ture.<br > The present volume is the culmination of years of research in<br > human genetics， many of the concepts having evolved gradually as ix<br ><br >