Chapter Seventeen is the section of this excellent book that you want to refer to, which discusses the way that Polynesia and the islands in the Pacific were inhabited. A very interesting point that Diamond establishes is the way that the population of Java, most other Indonesian islands and the Philippines is actually rather homogeneous in terms of the languages that they speak and their skin colour. Note what Diamond observes about the linguistic similarities:
Their languages are equally homogeneous: while 374 languages are spoken in the Philippines and western and central Indonesia, all of them are closely related and fall within the same subfamily... of the Austronesian language family.
Diamond concludes that such evidence strongly suggests that Southeast Asians or South Chinese people spread through these islands, replacing the former inhabitants on the whole. This evident occurred recently due to the way that skin colour has not changed. The diversity in languages points to the way that no system of unity was established between these different islands.