in their structure, in such case I grant that we could deduce nothing from domestic varieties in regard to species. But there is not a shadow of evidence in favor of this view; to assert that we could not breed our cart and race horses, long and short horned cattle, and poultry of various breeds, and esculent vegetables, for an unlimited number of generations, would be opposed to all experience."[8] Darwin recognizes, in instinct, the possibility for the play of a certain amount of imitation, as also of intelligence and experience,[9] though denying to these the range attributed to them by Wallace. And summing up his theory in the essay given by Romanes, he writes: "It may not be logical, but to my imagination it is far more satisfactory, to look at the young cuckoo ejecting his foster brothers, ants making slaves, the larvae of the ichneumidae feeding within the live bodies of their prey, cats playing with mice, otters and cormorants with living fish, not as instincts specially given by the Creator, but as very small parts of one general law leading to the advancement of all organic bodies--Multiply, Vary, let the strongest Live and the weakest Die." It will thus be seen that Darwin, while confessing a disability to account for the origin of _Instinct_,--beginning with some form of instinct as already existent, just as he begins with life as already existent,--does advance some perfectly definite views as to the probable origins of _instincts_,--namely, preservation, in the struggle for existence, of numerous slight but profitable variations. The assertion of the inadequacy of habit to account for the origin of more complex instincts, as in the case of the hive-bees, when compared with the subsequent explanation, in the same connection, of the rise of these very instincts partly by habit acquired from experience and imitation, partly by accidental modifications of simpler instincts, both taken advantage of by natural selection,--would seem to limit the term "habit," as here used, to modes of action acquired during the life of the individual; this interpretation of the word being confirmed by the additional phrase "in one generation." But here, as everywhere in Darwin's work, an unknown quantity appears--namely, the cause of variation; _i.e._ of the differences, or tendency to differ, of offspring, from the parental type.