even ratio) no _one_ could be tabulated in preference to another. This however is an example in which the rate of alteration (so far as colour is concerned) is _equal_; and one therefore in which the extreme end of the series can be alone singled out as _the_ aberration to be specially noticed. It sometimes occurs that, between the two extremes, there are several nuclei, or centres of radiation, to which the name of varieties may be legitimately applied,--inasmuch as they may possess a series of characters which do not, all, in combination, progress evenly; and which consequently stand out as it were, to as certain extent isolated, from the remainder. As a corollary arising out of these remarks, it would seem to follow that even small differences _should be regarded as specific ones_ so long as the intermediate links have not been detected which may enable us to refer them to their nearest types. In a general sense, I believe that it would be proper to do so: nevertheless there are instances, the results, for example, of isolation, in which _abrupt_ modifications may be _a priori_ looked for; and in which our judgment must be regulated by our knowledge of the local circumstances which may be reasonably presumed to have had some influence in producing them. The consideration of these, however, and other kindred questions, must be deferred to a subsequent chapter of this work. CHAPTER II. FACT OF VARIATION. It is scarcely possible to survey the members of the external world around us without being struck with the instability with which everything is impressed. The very shadows, as they pass, leave a moral lesson behind them on the mountain-slope, which the student of Nature would do well to contemplate. Whatever be our preconceived ideas of the "immutability of the universe," from first to last the same truth is re-echoed to our mind,--that here all is change. Organic and inorganic matter are alike subjected to renovation and decay; and, dependent on that general law, _variability_ within specific limits would seem to be an almost necessary consequence. In the animal and vegetable kingdoms, this principle of fluctuation is peculiarly apparent; and not more surely do the winds of heaven ruffle the forests over which they rage, than does the ebb and flow which is perpetually going on amongst created things mar their boasted constancy. The _fact_ of aberration, to which we would briefly allude in this chapter, requires but little comment; it is patent _a priori_. As a matter of experience, every observer who has spent a week in the field of Nature knows it to exist. However difficult it may be, in some instances, to distinguish aright between species and varieties, as rigidly defined, there is an instinct within us which often recognizes the _latter_, even at first sight, as unmistakeably such: and in these cases, a well-educated eye, although of course occasionally deceived, will not often be found to err.