Even less reliable are the opinions of speculative biologists who by the use of analogy--that is, by examples of hybridization of plants and animals--try to throw light on the subject of racial intermarriage. In general, the assertions of these biologists agree that the intermixture of races too far apart is undesirable because it results in a breakdown of the inherent characteristics of each, but that the combination of races slightly different is more desirable than intra-racial marriage because it tends to invigorate the stock. To this extent, opinions concur; but as to the question what races may be considered similar and what races different they begin to disagree. Most of them divide the human races by the color of the skin, and state that the case of the black and white races is that of extreme intermixture, and cite that between two white races as a desirable one. When they are pressed to pass a verdict on the result of mixture between the yellow and white races, most of them give only vacillating replies, as in the following extracts: Yellow-white amalgamation may not be fraught with the evil consequences in the wake of the yellow-black and the white-black crosses. At the same time, it should be pointed out that the Caucasians and the Mongolians are far apart in descent, and that the advantages to be gained by either in this breaking up of superior hereditary complexes developed during an extended past are not clear.[46] Professor Castle is more precise in his assertion. He says: Mankind consists of a single species; at least no races exist so distinct that when they are crossed sterile progeny are produced. Offspring produced by crossing such races do not lack in vigor, size, or reproductive capacity.... Racial crosses, if so conducted as not to interfere with social inheritance, may be expected to produce on the whole intermediates as regards physical and psychic characters.[47]