FIG. 71. Some chipmunks of California, showing distinct species produced through (From nature, by William Sackston Atkinson.)

isolation.

A

but not derived from one another by direct lines of descent. closer study indicates that some of them "came from closely related forms in remote geographic areas, others from antecedent forms now extinct, and not more than three or four from species still inhabiting the region."

The nature of any fauna bears an immediate relation to the barriers, geographic, climatic, topographic, or bionomic, which may form its boundaries. By bionomic barriers we mean any condition of any sort which may check free interbreeding, or which may tend to cause divergence within a species. A thickly peopled level area may be in this sense a barrier, because it prevents the animals on the one side of the arca from interbreeding with those on the opposite side. If the two extremes have diverged to become different species, the individuals in the middle area, whose presence in a sense constitutes the bionomic barrier, are usually variously intermediate in the characters and habits which they possess.

Whenever the individuals of a species move evenly over an area, its members freely interbreeding, the character of the species remains substantially uniform. Whenever freedom of movement and consequent freedom of interbreeding is checked, the character of the species is rapidly altered. It is changed even though external conditions seem to be absolutely identical on both sides of the barrier, and if there is no visible distinction in the original stock on the two sides. Presumably, there are subtle differences in the environment, producing changes in the process of selection and adaptation. Doubtless, there are differences equally subtle produced by the processes of variation and their repetition by inheritance.

The pregnant phrase of Dr. Coues applies in these cases: "Migration holds species true: localization lets them slip." In other words, free interbreeding swamps incipient lines of variation, and this in almost every case. On the other hand, a barrier or check of any sort brings a certain group of individuals together. These are subjected to a selection different from that which obtains with the species at large, and under these conditions new forms are developed. This takes place rapidly when the conditions of life are greatly changed, so that a new set of demands is made on the species, and those individuals not meeting it are at once destroyed. The process is a slow one, for the most part, when the barrier in question interrupts the

flow of life without materially changing its conditions. But this is practically a universal rule: A barrier which prevents the intermingling of members of a species will with time alter the relative characters of the groups of individuals thus separated. These groups of individuals are incipient species, and each may become in time an entirely distinct species if the barrier is really insurmountable. In the great water basin of the Mississippi, many families of fish occur and very many species are diffused throughout almost the whole area, occurring in all suitable waters. Once admitted to the water basin, each one ranges widely and each tributary brook has many species. In the streams of California, mostly small and isolated, the number of genera or families is much smaller. Each species, unless running to the sea, has a narrow range, and closely related species are not found in the same river. The fact last mentioned has a very broad application and may be raised to the dignity of a general law of distribution.

Given any species in any region, the nearest related species is not likely to be found in the same region nor in a remote region, but in a neighboring district separated from the first by a barrier of some sort, or at least by a belt of country, the breadth of which gives the effect of a barrier.

Always the species nearest alike in structure are not found together nor yet far apart, and always a check to interbreeding lies between. Where two closely allied forms are not found to intergrade, they are called distinct species. If we find actual intergradation, the occurrence of specimens intermediate in structure, the term subspecies is commonly used for each of the recognizable groups thus connected.

Widely distributed across the United States and from southern Canada to Arizona, we have the yellow warbler, Dendroica astiva. This bird is chiefly yellow, olive on the back with chestnut streaks on the sides, the tail feathers colored like the body, and without the white spot on the outer feathers shown in most of the other wood warblers composing the genus Dendroica.

The yellow warbler throughout its range is very uniform in size and color. Its nearest relative differs in having a shade less olive on the back and the brown streaks on the sides narrower. This form is found in the Sonoran region, and, as along the Rio Grande it intergrades with the first, it is called

a subspecies, Dendroica æstiva sonorana. Further south, in central Mexico, this form runs larger in size and is recorded as Dendroica astiva dugesi. Northward, through to Alaska, we have an ally of the parent bird, but smaller and still more greenish. This is Dendroica æstiva rubiginosa.

In the West Indies, the golden warblers migrate not from north to south, but from the shore to the mountains, and, possibly in consequence of the less demand of flight, the wing is shorter and more rounded, while the tail is longer. As these forms do not clearly intergrade with those of the mainland, and, for the most part, not with each other, they are held to represent a number of distinct species, although doubtless derived from the parent stock of Dendroica æstiva. Some of these West Indian forms are relatively large, the wing more than five inches long, and the longest known of these, the type of the species for this reason, found in Jamaica, is called Dendroica petechia. On the island of Grand Cayman is a similar bird, a little smaller, Dendroica auricapilla. Of a deeper yellow than petechia, and equally large, is the golden warbler of the Lesser Antilles ranging from island to island, from Porto Rico to Antigua. This form, first known from St. Bartholomew, is Dendroica petechia bartholemica. A smaller bird, a little different in color, takes its place in the Bahamas. This is Dendroica petechia flaviceps.

In Cuba, the golden warbler is darker and more olive, with other minor differences from the form called bartholemica, of which it may be the parent. This is Dendroica petechia gundlachi. A similar bird, but with the crown distinctly chestnut, is Dendroica petechia aureola, the golden warbler of the Galapagos and Cocos Islands, off the coast of Ecuador and Peru. Scattered over other islands are smaller golden warblers with the wing less than five inches long, and with the crown tawny red, as in aureola. These are known collectively as Dendroica ruficapilla, the type being from Guadeloupe and Dominica. More heavily streaked, with the crown darker in color, is the golden warbler of Cozumel, Dendroica ruficapilla flavivertex, and with very similar but with darker crown is Dendroica ruficapilla flavida, of the island of St. Andrews. Always, the nearest form lies across the barrier, and among these island forms the chief barrier is the sea. With a darker chestnut crown is Dendroica ruficapilla rufopileata, of the island of

Curaçao, and still darker bay is the crown of Dendroica ruficapilla capitalis, the golden warbler of the Barbadoes.

Still other golden warblers exist, with the chin and throat chestnut as well as the crown. One of these, olive green on the back, is Dendroica rufigula, of Martinique. The others are more yellow. One of these, with the sides heavily streaked, inhabits the isthmus region, Dendroica erythacoides, called a distinct species, because no intergradations have been made out. Another, more faintly streaked, replaces it on the Atlantic coast from Yucatan to Costa Rica, Dendroica bryanti, while the Pacific coast, from Sinaloa to Costa Rica, has another form, with still fainter markings, Dendroica bryanti castaniceps. An extreme of this form with the throat and breast tawny, but not the crown, is found in Jamaica again and is known as Dendroica eoa. In this case, which is one typical of most groups of small birds, the relation of the species to the barriers of geography is so plain as to admit of no doubt or question.

Given the facts of individual fluctuation and of heredity, it is manifest that while natural selection may produce and enforce adaptation to conditions of life, the traits which distinguish these species bear little relation to utility. The individuals which, separated from the main flock, people an island, give their actual traits to their actual descendants, and the traits enforced by natural selection differ from island to island. If external conditions were alike in all the islands the progress of evolution would perhaps run parallel in all of them, and the only differences which would persist would be derived from differences in the parent stock. As some difference in environment exists, there is a corresponding difference in the species as a result of adaptation. If great differences in conditions exist, the change in the species may be greater, more rapidly accomplished, and the characters observed will bear a closer relation to the principle of utility.

Doubtless, wide fluctuations or mutations in every species. are more common than we suppose. With free access to the mass of the species, these are lost through interbreeding. Isolate them, as in a garden, or an enclosure or on an island, and these may be continued and intensified to form new species or Any breeder or any horticulturist will illustrate this. It is not claimed that species are occasionally associated with physical barriers, which determine their range, and which

races.