Darwin remarks that this "advantage of diversification of structure in the inhabitants of the same region is, in fact, the same as that of the physiological division of labour in the organs of the same body.

No physiologist doubts that a stomach adapted to digest vegetable matter alone, or flesh alone, draws more nutriment from these substances.

So, in the general economy of any land, the more widely and perfectly the animals and plants are diversified for different habits of life, so will a greater number of individuals be capable of there supporting themselves."[39] _The most closely allied Species inhabit distinct Areas._ One of the curious results of the general action of this principle in nature is, that the most closely allied species--those whose differences though often real and important are hardly perceptible to any one but a naturalist--are usually not found in the same but in widely separated countries.

Thus, the nearest allies to our European golden plover are found in North America and East Asia; the nearest ally of our European jay is found in Japan, although there are several other species of jays in Western Asia and North Africa; and though we have several species of titmice in England they are not very closely allied to each other.

The form most akin to our blue tit is the azure tit of Central Asia (Parus azureus); the Parus ledouci of Algeria is very near our coal tit, and the Parus lugubris of South-Eastern Europe and Asia Minor is nearest to our marsh tit.