I am currently reading the King James Version of the Bible and am slowly getting used to the text-—English is my second language. I then wondered with what ease would I be able to understand the English of certain time periods, had I a time machine. I read some snippets from other historical texts and experienced the following:
Shakespeare's plays (late 1500s): I need to diligently read the text and especially focus on the word order and verb formations to understand the meaning of the text. I also need to Google a lot of words and phrases. Then, even after all my effort, I still struggle to comprehend most of the dialogue and the tone thereof.
King James' Bible (1611): I find it quite a bit easier than Shakespeare, but I still need to diligently read the text and cross reference a lot of things with my Afrikaans Bible.
The Pilgrim's Progress (1678): I find it a lot easier to read than all of the above, although still slightly awkward. I sometimes mentally stutter when I come across a archaic verb formation (e.g. the -eth third person singular form) or archaic words from my passive vocabulary. I also have to consult a dictionary every now and again.
Gulliver's travels (1726): It almost "reads like a book". The archaic verb formations are sparsely spread and easy to understand. (E.g. I barely noticed the following: "And if any traveler hath a curiosity"). I still find it handy to keep a dictionary nearby.
On the Origin of Species (1859): If someone gave me a book that is written in the exact same style and vocabulary as The Origin of Species, and told me it was written yesterday, it would raise no suspicion.
Is the above sample texts reasonable representations of their respective times? Or are some texts considered far more informal than others?
If my observations somewhat reflect the language of the respective times, then why is there such a vast transformation between the language just prior to The Pilgrim's Progress and the language in Gulliver's travels? Why does it seem like English grammar did not change so rapidly during the periods that followed?