2. Where two contiguous temperature-regions come in contact, there is no sharp line between; on the contrary, they _shade gradually_, almost imperceptibly, into one another, the ranges of species overlapping and interpenetrating, and the two species coexisting on the borders of their ranges. This is represented by the diagram (Fig. 64), in which the horizontal lines represent the north and south ranges of species of two groups, A and B, separated by the dotted line. 3. Species also pass out gradually on the borders of these ranges and others come in gradually, so _far as number and vigor of individuals are concerned_. If _a a′_ and _b b′_ (Fig. 65) represent the north and south range of two species, and _b a′_ their overlap or area of coexistence, then the height of the curves A and B will represent the number and vigor of the individuals in different parts of the range. [Illustration: FIG. 65.] 4. While, therefore, there is a shading of contiguous groups into each other by overlap of species-ranges; while there is also a gradual passing out of species so far as number and vigor of individuals is concerned, yet, in _specific characters_ we observe usually no such gradation. Species seem to come in on one border with all their specific characters perfect, remain substantially unchanged throughout their range, and pass out on the other border, still the same species. In other words, one species takes the place of another, usually by _substitution_, not by _transmutation_. It is _as if_ species had originated, no matter how, each in its own region, and had spread in all directions as far as physical conditions and struggle with other species would allow. This important subject will be more fully discussed later. 5. We have thus far spoken of species as limited by temperature alone, but they are limited also by _barriers_. If, then, there be an east and west barrier, such as a high mountain-range, or a wide sea or desert, there will be no shading or gradation of any kind, because the barrier prevents overlapping, interpenetration, and struggle on the margins. For example: The species north and south of the Himalayas, or north and south of Sahara, are widely different. It is, again, _as if_ they originated each where we find them and spread as far as they could, but the physical barrier prevented mingling and shading. 6. There are temperature-regions south as well as north of the equator. Now, although the climatic conditions are quite similar, the species of corresponding temperature-regions north and south are wholly different. It is, again, as if they originated where we find them, and were kept separate by the barrier of tropical heat between. If carried over, they often do perfectly well. _Continental Faunas and Floras._