heredity. Although we can rely in our theory building on the fact that no two individuals are exactly alike, yet we can equally. certainly rely on the fact that the offspring of any individual will be much more like other individuals of the species to which the parent belongs than like individuals of other species, and also, in the main, more like the parent than like other individuals of the same species. Heredity is the name we use for expressing this fact of likeness of young to parent.

Some biologists seem to mean by heredity a force or dominating influence which brings about this likeness; while others use the word heredity to name rather the processes which are gone through with by the young in becoming, in its total development, like the parent. The essential connotation of the word is, however, simply the fact that this likeness does exist and that we may rely on its continuing to occur. So that when the struggle for existence weeds out, if it does, those individuals of a too abundant population which possess variations of disadvantage or of no special advantage, leaving those to survive and produce offspring which do possess specially advantageous or fit variations, the fact of heredity permits us to assume the almost certain perpetuation of these advantageous variations by insuring their reappearance in the offspring of the "saved" individuals. Thus while we may liken the causes that produce ever-appearing variations to a centrifugal force making for difference and instability, heredity (if used as the name for the causes that produce likeness) may be conceived as a centripetal force, making for stability and

sameness.

But at least one other factor seems to be necessary in species-forming and that is the factor of isolation, separation, or segregation, as it is variously named. By this is meant that those individuals showing similar variations must in some way be segregated, made to live and breed together, in order that the particular variations (which from the point of view of the student of species-forming may be called also the particular varietal differences that are to become in time so developed and fixed as to be true species differences) may be maintained.

For it is obvious that if an individual possessing certain particular variations mate with another of its species possessing different variations, the offspring of this union will likely not possess in pure form the variations of that particular parent

we are for the moment interested in. The offspring may show a blend of the different characters of the parents, or a mosaic of them, or may show the characters of either one alone, or, indeed, characters of wholly new type. The important thing is, however, that there is no certainty-indeed there is almost certainty of the opposite-that any particular variation will be fostered and fixed if miscellaneous interbreeding is allowed. So that a segregation of individuals having certain common variations or varietal characters is necessary for the perpetuation of these characters.

Now the most usual way, probably, in which this segregation or isolation is brought about is by topographic or geographic barriers; a group of individuals gets isolated from others of their species by some physical barrier, and the variations that appear among them, due often to some cause incident to the special locality and hence common to all of them, are readily preserved and fostered by the enforced breeding among themselves. But such an isolation may conceivably be brought about in several other ways, and observation has shown that probably in some cases so-called biologic isolation occurs, that is, that a restriction of miscellaneous interbreeding among individuals of one species, and an enforced selective breeding among certain ones possessing certain variations or differences in common, does really obtain. Such isolation is also called physiologic, or sexual, isolation.

Many biologists, and the number of them has increased rapidly in the last few years, due primarily to the activity and leadership of the botanist de Vries (Amsterdam), believe that species-forming is achieved without the aid of the selection factor; that the actual production of species is a function of variation ("mutation" the special kind of variation efficient in species-making is called), and that the influence of selection is only of a more remote and generally restraining, and thus directive, nature. Such biologists may be said to believe in species-forming by heterogenesis or saltation, as contrasted. with species-making by slow, gradual transmutation. And de Vries and his followers have adduced a few apparently undeniable examples of species-forming by heterogenesis. At least this influence seems to have produced forms to all intents and purposes apparently similar to natural species. So the particular kind of variation called mutation, which is the

basis of this sort of species-making, must be added to our list of evolution factors.

Some other biologists, of whom the botanist Nägeli, the zoologist Eimer, and the paleontologist Cope are representatives (all three of these men, however, having evolution theories and beliefs distinct and peculiar to each), believe in what may be called orthogenetic evolution. That is, that the lines of descent are determined by the appearance of certain special determinate lines or tendencies of variation or change, this nonfortuitous and determinate variation being itself determined by certain causes either (in Nägeli's belief) inherent in life, or (in Eimer's belief) extrinsic to life but imposed upon it, as for example the influence of climate, etc. So that orthogenesis or determinate variation should also find a place in any list of assumed evolution factors.

While it is apparent that variation is ever present and also apparent that heredity or the fact of likeness is always ever to be relied on, the exact relationship or correlation of these two evolution factors is not so apparent. That heredity often. preserves or perpetuates variations after they have occurred is well proved, but it is also proved that some variations appearing in the parent are not handed on to the parent's offspring, nor indeed to any future generations of the line. And the general answer to the natural query raised by this condition is that variations which are congenital or blastogenic, that is, are determined at birth for it (although they appear of course only after development), are heritable (that is, will be passed on from parent to offspring); but that variations or modifications acquired during the lifetime of the individual, that is, those which are impressed on it by extrinsic influences during its "growing up" or development, will not be heritable. Thus such modifications in body parts as may be produced by use or disuse, or by other functional stimulation or lack of it, changes caused by mutilation or disease, etc., are believed by most biologists to be non-heritable. Hence it is that only the congenital variations are looked on by these biologists as of importance in the matter of species-forming. Yet the whole pre-Darwinian evolution theory of Lamarck was founded on the assumption that the modifications in individuals due to use, disuse, and other functional stimulation, in a word that all body change and adaptation, all characters acquired during the lifetime of an individual,

can be, in some degree at least, handed on by inheritance to the offspring. And there are to-day many Lamarckian evolutionists. So that in our list of possible evolution factors the so-called Lamarckian factor should not be omitted. And in connection with it may be considered, by and large, the immediate influence or non-influence on individuals and on species of all environmental conditions; and particularly the results of such influence during development. In fact the study of development has come largely to be a study of the actual influences or factors that determine and guide growth, instead of one purely descriptive and comparative as in the older days of embryological study. Some of these factors are apparently strictly inherent in the protoplasmic germ cells and in the embryo substance: others are as obviously extrinsic or epigenetic. And the determination of the relative influence and power of these two sets of developmental factors and of the various members of each set is one of the most eagerly worked-at problems of modern biological study.

Finally, the general term adaptation should be mentioned in any list of evolution factors; although it is more usually looked on, not as a factor, but as an evolution problem and indeed one of the greatest of the problems. Adaptation is precisely one of the things evolutionists are trying to find the causes or causal factors of. But nevertheless the adaptability of life stuff, its plasticity and capacity of advantageous reaction, is, to many biologists, a fundamental fact in organic nature, like gravitation or chemical affinity in inorganic nature: a thing basic and inexplicable, and in itself a factor whose consequences are to be determined but not further to be questioned as to their cause.

CHAPTER V

NATURAL SELECTION AND THE STRUGGLE

FOR EXISTENCE; SEXUAL SELECTION

The tendency to regard natural selection as more or less unnecessary or superfluous which is so characteristic of our day, seems to grow out of reverence for the all-sufficiency of the philosophy of evolution, and pious belief that the history of living things flows out of this philosophy as a necessary truth or axiom.-BROOKS.

La selection naturelle est un principe admirable et parfaitement juste. Tout le monde est d'accord aujourd'hui sur ce point. Mais où l'on n'est pas d'accord, c'est sur la limite de sa puissance et sur la question de savoir si elle peut engendrer des formes specifiques nouvelles. Il semble bien demontre aujourd'hui qu'elle ne le peut.

--DELAGE.

Of all the various factors of organic evolution the one which has been most relied on as the great determining agent is that called Natural Selection, the survival of the individuals best fitted for the conditions of life, with the inheritance of those species-forming adaptations in which fitness lies. The primal initiative is not in natural selection, but in variation, germinal and individual. This may be slight variation (fluctuation) or large deviation (saltation), but in any case all difference in species or race must first be individual. The impulse to change, once arisen, is continued through heredity. From natural selection arises the choice among different lines of descent, the adaptive tending to exclude the non-adaptive, while traits which are neither helpful nor hurtful, but simply indifferent, may be borne along by the current of adaptive characters. Finally separation or isolation tends to preserve a special line of heredity from being merged in the mass which constitutes the parent stock or species.

Without individual variation, no change could take place; all organisms would be identical in structure. Without heredity,