(archegonia) appear. In the first stage of its development it consists of a large number of free nuclei; in the second stage walls appear and a tissue is formed; and in the last stage this tissue grows and finally produces the archegonia with their egg. The constant tendency throughout the whole group is to produce the female sex organs earlier and earlier in the history of the gametophyte. A series can be arranged illustrating the appearance of the sex organs at what might be called the mature stage of the gametophyte, at one extreme; then their appearance at earlier and earlier stages of the tissue development, until they appear with the first formation of walls; and finally, at the other extreme, the eggs appear at the stage of free nuclei, so that no sex organs are formed. This progressive slipping back of the egg in the ontogeny of the gametophyte holds no relation to any advantage that can be detected. Certainly it holds no relation to any advantage in fertilization, for that is a prolonged process among Gymnosperms, and the pollen tube containing the sperms may live for a season or two in the tissues of the ovule. Taking the group as a whole, this is not a sporadic change, occurring here and there; but the two extremes I have given are the two extremes of the Gymnosperm phylum. This kind of progressive change is beyond the reach of experiment, and its explanation is beyond the reach of imagination as yet. The same kind of progressive change is shown also in the embryo of Gymnosperms. In the most primitive condition, the first stage of embryo formation is extensive free nuclear division within the fertilized egg; after this, walls are formed and the egg becomes filled with tissue, the proembryo. Throughout the Gymnosperm series there is a steady reduction of the amount of free nuclear division, and with it a reduction of the amount of proembryonic tissue, so that finally it occupies a very small portion of the fertilized egg. All this change has taken place further from outside influences than the change in the gametophyte, for the embryo is imbedded in the gametophyte. It may be claimed that these are not the characters that taxonomists use in distinguishing species. This is true, but they are just the characters that distinguish great groups, and represent the advancement of the plant kingdom as a whole. It so happens that both of the progressive changes noted as occurring among Gymnosperms culminate among Angiosperms. The male gametophyte of Gymnosperms shows a similar progressive change, not so steady, but none the less evident. Its few cells are contained within the resistant wall of the pollen grain which produces it. In the more primitive condition the vegetative cells are variable in number, but evident; but there is a persistent tendency to eliminate them, which reaches comple tion in certain Gymnosperms, and is a constant feature of Angiosperms. It may be said that in all these cases we are dealing with structures that have ceased to be useful, and therefore are being gradually eliminated. No one can say how useful they are, but no one can deny that they are functional. But there is a striking illustration of another sort among Gymnosperms. The suspensor is a conspicuous organ of the embryo in this group, with a development apparently out of all proportion to its usefulness. In fact, it is a most exaggerated structure, often becoming closely coiled on account of its extreme length. One would suppose that this would be the first structure eliminated, or at least curtailed, if usefulness determines suppression. But the suspensor of Gymnosperms shows no symptom of suppression throughout the whole group, and still among the heterosporous Pteridophytes below and the Angiosperms above, where the same conditions prevail, it shows no such unusual development. Several illustrations could be taken from Gymnosperms, all of them fundamental in the structure and progress of the group, and none of them in use by taxonomists. My claim is that it may be one thing to pass from species to species within the limits of a small natural group; and a very different thing to pass from one great group to another. I do not doubt that the characters of a genus may have been juggled in a variety of ways to form what we call its species, and that one of these ways may have been Natural Selection, with or without adaptation. Our problem, however, includes more than the origin of species. All of our observation and experimental work in this field is immensely important in demonstrating the theory of descent, and in showing how the final diversity of species is reached; but the methods for securing this final diversity may not apply and probably do not apply to the establishment of the assemblages of different characters that distinguish the great groups, and that any study of phylogeny shows to have been wrought out by steady and progressive change through all imaginable changes of environment. Species have been likened to the individual waves that appear on the surface of a choppy sea; if so, the deepseated changes to which I refer, and which phylogeny makes so evident, may be likened to the great oceanic currents, whose movement and direction proceed with no relation to the choppy surface. ISOLATION AS A FACTOR IN BY DAVID STARR JORDAN By isolation, segregation or separation as a factor in evolution, we mean the failure of a portion of one group or species to interbreed freely with the rest of its kind. Such failure is due to the presence of some barrier which prevents free intermingling of individuals or to some condition or group of conditions which sets certain individuals off from the mass of their kind. Through separations of this sort race distinctions arise, and in time by the same means the more profound modifications which mark what we call species. The occasion of divergence in most cases is found in geographical separation, the "raümliche Sonderung," on which such strong emphasis has been justly laid by Moritz Wagner. It may again be a separation of some other kind, as segregation, through the occupation of different tracts within the same general area, or seasonal separation, as when flowers bloom or animals mate at different times of the year. There are also forms of physiological segregation. Self-fertilized plants mate with their neighbors irregularly or by chance, the pure species standing alongside of |