During the last few years a number of papers have been published by entomologists in connection with the hypothesis known as the “ Hopkins host-selection principle.” This principle, as defined by Dr. A. D. Hopkins himself, is that an insect species that breeds in two or more hosts will continue to breed in the host to which it has become adapted. Thus, according to this author, the mountain pine beetle, Dendroctonus monticola, will destroy mountain pine, yellow pine, lodgepole pine and sugar pine, but if it becomes established in one species of pine through many generations, the beetles on emergence show a decided preference for the species in which they have bred and will not, in fact, attack any other. In 1922 Craighead published a paper giving the results of experiments carried on during a number of years with about a dozen species of Cerambycids. He states that in practically all the species studied the adults show a marked predilection for the host in which they have fed as larvae, provided that they are not deterred by other factors. Continued breeding in a given host is said to intensify the preference for that host. With some beetles whose larvae can be transferred to another species of plant and successfully reared therein, this association with the new host for a year, or even less, during the latter part of the larval life is said to determine a preference for this in the resulting adults. The author believes that his experiments may indicate the mode of origin of certain closely related species or varieties. The conclusions at which he has independently arrived were long ago suggested by Walsh. That new forms do not thus arise more rapidly in Nature, Craighead considers to be due to the high mortality of the young larvae in the new hosts.