The directed movement of substances, in membrane-bounded vesicles, from the early sorting endosomes to the late sorting endosomes; transport occurs along microtubules and can be experimentally blocked with microtubule-depolymerizing drugs. [ http://www.ncbi.nlm.nih.gov/pubmed/29980602 https://www.worldcat.org/search?q=bn%3A0815316194 ]