Sentence:  [The] Caucasian race includes two sub-races, — the Xantho-chroic and Melanochroic of Huxley. The seat of this race is Europe, northern Africa, and southwestern Asia, its linguistic division being into <WSD>Aryans</WSD>, Semites, and Hamites. 
Meaning of the word marked with <WSD> tags:  A subdivision of the Caucasian racial and linguistic grouping, when that grouping is defined as consisting of Aryans, Semites, and Hamites.