By the last quarter of the 18th century magazines and fashion plates are being constantly produced. While the fashion plates are still costly and time consuming to make (therefore often the style was out too fast to sell enough in time), descriptions could be printed easily and are found not only in magazines discussing fashionable life, but even in regular newspapers or even just a letter from Cousin Beth in London. Even looking back as historians we can pinpoint things like during the summer of 1778 blue layloc (lilac) was a very fashionable color. It was said that the popular cap style "changed with the moon" and ever fluctuating fashion was complained about.

It'd be hard to consider that time to be the "modern fashion industry", however. The method of purchasing clothing, other than unfitted basics, was to have it custom made by a professional. The customer buys the fabric and notions, decides the style, and generally has the control over how the garment looks and fits (or can have the tailor/mantua maker decide if they don't want to). More pre-made garments are appearing at the very end of the century, but it's really not common to buy a fitted, fashionable garment in that manner until the later half of the 19th century. So, while fashion is certainly originating somewhere and changing constantly, it's source isn't really an industry in the way we think of it. By the time there are names of designers like Worth there's already a standard for seasonal or even monthly new fashions.