If real species, sterile one with another, differently adapted, now inhabiting different countries, with different structures and instincts, are admitted to have common descent, we can only legitimately stop where our facts stop. Look how far in some case a chain of species will lead us. <This probably refers to the Crustacea, where the two ends of the series have "hardly a character in common." _Origin_, Ed. i. p. 419.> May we not jump (considering how much extermination, and how imperfect geological records) from one sub-genus to another sub-genus. Can genera restrain us; many of the same arguments, which made us give up species, inexorably demand genera and families and orders to fall, and classes tottering. We ought to stop only when clear unity of type, independent of use and adaptation, ceases. Be it remembered no naturalist pretends to give test from external characters of species; in many genera the distinction is quite arbitrary{177}. But there remains one other way of comparing species with races; it is to compare the effects of crossing them. Would it not be wonderful, if the union of two organisms, produced by two separate acts of Creation, blended their characters together when crossed according to the same rules, as two races which have undoubtedly descended from same parent stock; yet this can be shown to be the case. For sterility, though a usual <?>, is not an invariable concomitant, it varies much in degree and has been shown to be probably dependent on causes closely analogous with those which make domesticated organisms sterile. Independent of sterility there is no difference between mongrels and hybrids, as can be shown in a long series of facts. It is strikingly seen in cases of instincts, when the minds of the two species or races become blended together{178}. In both cases if the half-breed be crossed with either parent for a few generations, all traces of the one parent form is lost (as Koelreuter in two tobacco species almost sterile together), so that the Creationist in the case of a species, must believe that one act of creation is absorbed into another! {177} Between the lines occur the words:--"Species vary according to same general laws as varieties; they cross according to same laws." {178} "A cross with a bull-dog has affected for many generations the courage and obstinacy of greyhounds," _Origin_, Ed. i. p. 214, vi. p. 327. {Illustration: Facsimile of the original manuscript of the paragraph on p. 50.} CONCLUSION.