Few observers can have failed to remark, that increased _altitude_ frequently corresponds, both in its fauna and flora, to a higher _latitude_; and that, consequently, if we ascend the mountains of a southern land, we shall be struck, at times, by the presence of a host of species which obtain at a lower level in more temperate zones. This is peculiarly traceable in the Madeira Islands,--which, from their subaustral position, and height (the loftiest peak of the central mass exceeding 6000 feet above the sea), afford a rich field to the student of zoological geography. Yet, though the degrees of mere heat and cold are such as to allow, in the two cases, species positively identical to flourish; we should surely anticipate some slight change from the different atmospheric conditions (especially when in union with other circumstances) to which they have been, through a lapse of ages, respectively exposed: it may be well therefore to inquire, whether experience does at all tend to strengthen what our reason has an _à priori_ inclination to endorse. It must be recollected however that, in the instances to which we would draw attention, _small_ aberrations are all that can be usually looked for, since climate _of itself_ does not appear to be very potent in its action. We should remember, also, that the boundaries of insect instability are restricted; and, although we would advocate freedom of development within limits which are more or less comprehensive according to the species, to pass beyond them would be confusion, and such as could result from a _lapsus Naturæ_ only, rather than from a power of legitimate variation.