Monique said:
And to add to that: it is really distracting to read a text with grammatical errors, they draw your attention from the content. The author of the book is excused btw, it seems that the rules for which/that were not yet established in 1909 (when the book was first published). I do find it strange that they did not correct the grammar in later editions.
Worse than being distracting, they can confuse the meaning of the sentence.
Any variation which is not inherited is unimportant for us.
That sentence is clearly wrong. It should be: "Any variation
that is not inherited is unimportant for us." "Is not inherited" is a conditional requirement of "variation" to make the statement true.
I cannot here give the details which I have collected and elsewhere published on this curious subject.
As mentioned above, this example is harder to assess, and is a good example of why one needs to use the correct word, "which" or "that."
The first option is: "I cannot here give the details, which I have collected and elsewhere published, on this curious subject."
In this case, the author is saying they can't give details. The clause "I have collected elsewhere and published," is not a required modifier of "details," but is additional information. Think of something that begins with "which" as an aside. It's extra information that doesn't change the meaning of the sentence. You should be able to write the sentence without that clause and still have the original meaning intact: "I cannot here give the details on this curious subject." (And oh, by the way, those details are collected and published elsewhere, in case you're interested.)
The other option is: "I cannot here give the details that I have collected and elsewhere published on this curious subject."
In this second example, the phrase "I have collected and elsewhere published on this curious subject" modifies "details." It is a required part of the sentence to clarify a specific type of detail that is not going to be included. The details that have not been published elsewhere, or that have not been collected by this author, can be given, but not those collected by this author and published elsewhere.
There are a few other grammatical curiousities in that second example. The author says he "cannot" give the details. This may be true if he has published the details previously and does not have permission from the original publisher to include them in this later publication. But, more likely, he has "chosen not to" give the details, perhaps because it would make the book overly long, or would be a tangent or distraction from the main point.
There is also some awkward construction in the order of the words in "I cannot here give..." and "...elsewhere published..." Those parts of the sentence would be better written, "I cannot give here.." and "...published elsewhere..."
It seems this author had a pretty lousy editor if they didn't catch grammatical errors that are this blatant.