When this book was published Goddard was Director of the Research Laboratory of the Training School at Vineland, New Jersey, for Feeble-minded Girls and Boys. Though this work drew wide attention to the problems of people with intellectual disability, the author later rejected the scientific methodology and the conclusions that he had used in this work. Later students of the data that Goddard used criticised Goddard for distorting the data to reflect his personal bias. Full text is available from psychclassics.yorku.ca at this link.