...any seminary history professor worth his or her salt...
BTW, I would recommend not sinking too many eggs in this basket. This is something that changes with the wind. Well, maybe not with the wind, but changes in ecclesiology may change one's interpretation of the history before them, as well as new historical discoveries that changes minds. B. H. Carroll, who started the Southwestern Seminary, held some form of idea similar to his brother's, and probably most Baptist seminarians in America held some sort of Baptist perpetuity view until the late 1800s. This gave way to the English Separatist origins view (e.g. McBeth), with a few holding Anabaptist kinship (e.g. Estep). Some today have begun to revise that into a sort of multiple streams view. For example, Nathan Finn:
The third approach to question of Baptist origins might be called a "convergence approach." A convergence approach attempts to get over the "either/or" divide among historians by totally reframing the question: what are the sources of the Baptist movement?...According to the convergence approach, there are multiple "tributaries" that flow into the "river" of 17th century English Baptists.(in A Better Paradigm for the Study of Baptist History?, page 8)
Finn goes on to name three tributaries: English Separatism, Continental Anabaptists, and pre-1525 immersionist groups. This makes more sense from a purely historical standpoint. The "nothing before 1640-something" smacks of an attempt toward redefining the distinctives of Baptists and melding their history to suit a certain viewpoint. It certainly was for William H. Whitsitt, which anyone can see from his diaries and private correspondence in
W. H. Whitsitt: The Man and the Controversy, by James H. Slatton. Carroll's
Trail of Blood is an attempt on the other side to wrangle bits of history to prove his ecclesiological viewpoint. To me, Finn's approach tries to assess the history on historical terms and just say this is what we can find in history right now, whether or not we can make it fit what we want it to fit.
Based on what I read in the New Testament I believe there have been throughout the centuries some Christians who were not Catholics that believed and practiced the simple faith of the Bible, who assembled together and practiced that faith. I cannot demonstrate that historically. That is, I can find some at different times, but can't always prove they were orthodox or that they were connected.
A History of Anti-Pedobaptism by Albert H. Newman is helpful along this line. (Copyrighted in 1896, no doubt it needs an update that encompasses discoveries since that time.)