Inspired by some recent perusal of Heritage Foundation immigration material and the scholarly background of some of their researchers, I found out about a fascinating 1923 book by Carl C. Brigham, A Study of American Intelligence, which took advantage of World War I draft data to provide policymakers with some alarming facts about the declining intelligence of the American immigrant pool due to a falling share of Nordic types and an influx of slavs and such. You can find the whole text of the book here, but this is from Robert Yerkes' introduction:
Mr. Brigham has rendered a notable service to psychology, to sociology, and above all to our law-makers by carefully re-examining and re-presenting with illuminating discussion the data relative to intelligence and nativity first published in the official report of psychological examining in the United States army. Far from belittling or casting doubt on the general reliability of the results contained in the report, he has essentially confirmed the major findings in the field fo his special inquuriy and has adduced new evidences of the trustworthiness and scientific value of the statistical methods used by military psychologists. His task has been arduous and difficult, involving an immense amount of tedious labor for mathematical calculations and critical study of the results. The volume which is the outcome of Mr. Brigham's inquiry, and which I now have the responsibility and satisfcation of recommending, is substantial as to fact and important in its practical implications. It is not light or easy reading but it is better worth re-reading and reflective pondering that any explicit discussion of immigration which I happen to know. The author presents not theories or opinions but facts. It behooves us to consider their reliability and their meaning, for no one of us as a citizen can afford to ignore the menace of race deterioration or the evident relations of immigration to national progress and welfare.
Brigham, it turns out, isn't worried about an influx of Mexicans so much as an influx of Italians. The book is primarily dedicated to presenting and explicating actual IQ test data rather than verbal argument, but you can get the key gist starting at around page 112 where he observes that "the fact that the average intelligence of the immigrants examined in the army who came to this country in each successive five year period since 1902 becomes progressively lower with each succeeding period indicates that an explanation of this phenomenon might be found in a change in the character of immigration."
So he delved more deeply into the statistical data and finds a large change in the late 1890s: "a very marked decrease in the proportion of the immigration from England and Germany, and a substantial decrease in the proportion of immigration from Scotland, Sweden, and Ireland. On the other hand, the proportion of immigrants coming from Austria, Russia, and Italy showed a marked increase at this time" (note that at the time Austria was an extremely large country with a mostly Slavic population).