What are the rules for the usage of which and that? I am reading a book that (<- ) I think is littered with this grammatical error. How should the words be used correctly and are the following sentences wrong? Any variation which is not inherited is unimportant for us. I cannot here give the details which I have collected and elsewhere published on this curious subject. When we compare the individuals of the same variety or sub-variety of our older cultivated plants and animals, one of the first points which strikes us is, that they generally differ more from each other than do the individuals of any one species or variety in a state of nature.