I'm currently reading a book on Gil Elvgren, who is arguably the greatest pin-up artist of the golden age of pin-ups. The nature of the golden age of pin-ups is that the genre worked side by side with the golden age of illustration, and in the section on Gil Elvgren's influences and contemporaries, it does in fact feature works by several of my favorites; Andrew Loomis (IMHO the greatest teacher by publication in representational and figurative art history) is prominent, as is the buzzword artist of the illustration genre, Norman Rockwell. It's also worth noting in this chain that Loomis is listed as a primary influence of Alex Ross.
One thing I can most definitely say for Andrew Loomis is that, in his book "Creative Illustration," he credits his major influences. Principle among them is Howard Pyle, and he reprints a document of Pyle's which meticulously outlines the exact scientific nature of how light works. Howard Pyle's paintings might not have been the most beauteous of the age (certainly nothing to scoff at though) but he ended up with a much greater influence than his contemporaries because of his contribution to artistic education.
Now the question arises: who taught Pyle? A quick glance over Wikipedia will show that artists are frequently documented with their noteworthy students, but less frequently with their noteworthy teachers. And more important than Howard Pyle, who can trace an artistic lineage all the way back to someone like Da Vinci? The next important idea (especially in terms of Pyle and Loomis, who specifically published as a form of teaching) is whether an artist was directly mentored by another artist, or just intensively studied them. Especially in our modern world, resources needed to STUDY an artist are legion, but it's rare to directly apprentice under James Bama, Alex Ross, or similarly impressive name-drops. That is, unless you pay out the ass for animationmentor.com's resumé of Lucasarts and Pixar veterans. As Jimi Hendrix said, let me stand next to your fire.
That being said, I think that due to technological proliferation (whether it be the ease of internet or revolution of industrialized printing presses) at around 1910 or so, it became increasingly plausible for a young artist to "study under" a historic artist simply by examining his works and artifacts. Alex Ross is the greatest example of this, especially with him listing Andrew Loomis as an influence. Did he study under Andrew Loomis? Technically no, but he was part of a generation whose realistic artists undoubtedly owned all of Loomis' books. Loomis, lacking mass produced instructional texts, represents a generation that needed hands-on instruction. Theyir primary methos of learning were apprenticeship, their local gallery, and (Leonardo's favorite) learning through good ol' fashioned study of nature.
The modern generation is different in my opinion. For one thing, we have a vast population of talented artistic teachers who yell at us to go study nature; I think we're more likely to do so just because it's said much more frequently. Second, we can tailor who we WISH were our influences, learn about these tailored influences with the ease of a google image search. I recently saw my first ever Walter Langley painting through the randomness of the internet. I instantly fell in love, looked up a bunch of his stuff, and plan to regularly return to his paintings in hopes he rubs off on me. Even doing this through the library is easier, as the internet lets me manually check out Langley-related books in a moment. And thirdly, the modern artist is different because we exist with the luxury of "Tutorial Culture." Do you want to be a matte painter? Go explore www.3dm3.com and, with a few days of tutorial crunching, you'll be closer. Want to be a rigger? Go to www.rigging101.com and, with a few days of tutorial crunching, you'll be closer. Want to find out who's way better than you, then ask them questions about where to go next? Go to CGtalk and explore their "best of" section--expect a major blow to your self esteem and a major step forward in your understanding of representational/cartoon art.
But the question still remains...who can trace their artistic family tree back to Leonardo Da Vinci? Can you?