I am assuming that the standards in the past were much loser than they are now and that the precise definitions of plagiarism evolved some time in the 20th century. Correct?
Coturnix is right about standards being different in the past, but it was much further in the past that he thinks. In ancient and medieval Europe, there was something of a double standard about plagiarism. Many authorless genres like religious texts were freely copied and incorporated into later works, "good writing" usually meant slavishly imitating a small number of respected authors (Cicero being the most important), and scholarship meant demonstrating mastery of the ancient greats. On the other hand, poets and playwrights have always jealously defended their words.
This began to change during the Renaissance when original scholarship became more respected and individual accomplishment was recognized in many more fields that it had been previously (for example, this is when painters began signing their works). Why this happened is an enormous subject that I won't go into right now. The point here is that, by the mid 1600s, accusations of plagiarism and stealing ideas were common in every creative field including the sciences. An accusation of stealing someone else's words or ideas and passing them off as your own was one of the worst insults imaginable and grounds for lawsuits and duels.
Just as a sidebar, the word "plagiarism," in the sense we use it today, first appeared in English in the various battles among Shakespeare and his peers. The mighty and majestic Oxford English Dictionary credits Ben Jonson with being the first to use it in print. The form they used was "plagiary," which is a Latin term for a type of kidnapper or illegitimate slaver.
The first English copyright law was passed in 1709. It had as much to do with protecting the rights of publishers against book piracy as it did with protecting the author's rights against unscrupulous printers, but author's rights developed very quickly. James Boswell, best known as Samuel Johnson's biographer, was a lawyer who argued one of the important cases over how long copyrights lasted for an author and his or her heirs (it was twenty one years at the time).
By the beginning of the nineteenth century, the concept and the law were very similar to what they are today. Even footnotes were being used in a form very similar to what they are today. What has changed since then has been the issue of enforcing copyrights across borders. Most European countries concluded agreements to prevent book piracy (we should probably thank Napoleon for making that possible by dramatically reducing the number of countries in Europe). The United States was the odd man out. We refused to give any protection to foreign authors and publishers until 1891 and we didn't sigh on to the Berne Convention until 1988.
The bottom line for my mammoth post is that Buel, Lansdell, MacLean, and Lyell were all operating under the same concepts and very similar law to what we have today.
In the spirit of the topic, let me declare that most of what I know about this subject comes from Thomas Mallon's 1989 book Stolen Words: Forays into the Origins and Ravages of Plagiarism.