not very great divergent modification. It is impossible to conceive a more beautiful illustration of the principles we have been trying to enforce. * * * * * Thus, then, undoubtedly all the phenomena of geographical distribution of species are most rationally explained on the principle of slow evolution--changes, different in different places, and increasing with the time of isolation and its completeness. =Objection.=--The only objection which can be raised against this view is the manner in which contiguous geographical faunas and floras pass into one another when they are _limited not by barriers but by temperature_. In passing from equator to poles, over continuous land, we of course pass through many successive faunas and floras, limited wholly or mainly by temperature. Now, if species are indeed indefinitely modifiable, then on the borders of contiguous faunas or floras, where one species disappears and another closely allied but adapted to a colder temperature takes its place, the one species (say the anti-evolutionists) ought to be gradually _transmuted_ into the other, so that all the gradations may be traced. But this is certainly not usually the fact. On the contrary, a species may indeed pass out gradually, and another come in gradually, so far as _number and vigor of individuals are concerned_; but, in _specific character_, they may be said, usually at least, to come in suddenly, with all their characters perfect, remain unchanged throughout their whole range, and pass out suddenly at its borders. Another species takes its place, overlapping in range and coexisting on the borders of both; this also continues unchanged, as far as it goes, and so on. The change from one fauna to another is apparently not by _transmutation_ of one species _into_ another by gradations, but by _substitution_ of one perfect species _for_ another perfect species. As a broad general statement, the condition of things is precisely such as would be the case if specific types were substantially immutable by physical conditions, but were originated in some inscrutable way (created) in the regions where we now find them, and have spread in every direction as far as physical conditions and struggle with other species would allow them--their ranges therefore interpenetrating and overlapping one another on their borders.