Popular Science Monthly/Volume 16/December 1879/The Genesis of Sex
By Professor JOSEPH LE CONTE.
THE subject on which I address you to-day is one which is still veiled in much obscurity—so much so, indeed, that it is barely alluded to by evolutionists, is not touched upon by physiologists, and is regarded by the popular mind, even the intelligent popular mind, as wholly beyond the possible ken of human science.
1. Defining the Subject.—In regard to the origin of sex there are two distinct yet closely-related questions: 1. The origin of sex in the history of the individual; 2. The origin of sex in the history of the organic kingdom. The one question is, What are the conditions which determine the appearance of the one or the other sex in the development of the embryo?" The other question is, "What is the process and what are the steps by which sex was developed and then gradually differentiated in the evolution of the organic kingdom?" The one is the genesis of sex in ontogeny; the other the genesis of sex in phylogeny. It is this latter question which I wish to bring before you to-day.
The two questions, however, though distinct, are yet closely related. The ontogeny is a rapid recapitulation of the main points of the phylogeny. As in the former, sex was developed out of a primitive sexless condition of the embryo, so in the latter the sexed condition so universal now among mature organisms was evolved out of a primitive sexless condition of the organic kingdom. In the ontogeny some of the conditions which determine sex have been determined and others surmised. In some animals, as, for example, in some insects and crustaceans, the fact of fertilization or non-fertilization determines with certainty the sex, as proved by the well-known observations of Siebold and others on parthenogenesis. In others it is probably the degree of maturity of the ovule at the moment of fertilization that determines it, as shown by the experiments of Cornaz under the direction of Thury. In still others, as, for example, in butterflies, it seems to be the kind and degree of nutrition of the larvæ, as shown by the observations of Mrs. Treat. In still others it may be the prepotency of the one parent or the other, or still other causes wholly unknown. In any case, however, the subject lies fairly within the domain of science; the conditions will eventually be discovered, and, being known, will be artificially arranged so as to determine the one sex or the other with certainty.
But this is not the question which now concerns us, for we have already discussed this in a previous lecture. We wish in this lecture to show that, in the history of the organic kingdom also, sex has been gradually evolved out of a primitive sexless condition, and if possible to catch some glimpses of the main steps of the process. The most important steps are indeed very obscure; but this is only because these are among the very earliest steps of evolution.
2. The General Law under which the Process falls.—Now, the law under which I wish to bring the process of evolution of sex is that most universal of all the laws of evolution, viz., the law of differentiation. We have already explained to you and illustrated in many ways how, from an almost unorganized condition, in which every part is like every other part, and each part performs in an imperfect manner all the functions necessary to life—how, I say, from this primitive generalized condition, the several organs were gradually differentiated, the several functions separated and localized, and thus the complex work of the body parceled out by division of labor, until in the highest organisms each part or organ has but one function to perform, and therefore does it thoroughly. You will observe that the final cause, the end to be attained, the raison d'être, in all this process is better work, a better result. Now, my object will be to bring the origin of sex under this general law—to show some of the steps, and that each step was attended with better results.
3. The Kinds and Grades of Reproduction.—You already know that there are two fundamentally distinct kinds of reproduction, viz., sexual and non-sexual—so distinct, indeed, that there seems to be no possible connection between them. But remember that not only are our distinctions in science far more trenchant than they are in nature, but also that the distinctions in nature now are far more trenchant than they were in early geological times. It is the peculiarity of modern science, under the guidance of the doctrine of evolution, that it loves to dwell upon the gradations rather than upon the distinctions—it seeks for the missing links which make the chain of nature continuous. Now, there are several grades of sexual as well as of nonsexual reproduction; and through these grades they closely approximate each other. For example: sexual reproduction consists essentially in the union of two different cells, the germ-cell and the sperm-cell, to form one cell, the ovum. It is in the most literal sense a union of diverse twain to form one flesh. These two cells may be called the sexual elements. This is all that is absolutely necessary to the idea of sexual reproduction, even though the two elements may be formed by the same organ. But, further, the two elements are usually elaborated by two distinct organs, viz., the ovary and the spermary. These are the essential sexual organs. When these two organs are found in the same individual, the condition is called bisexuality, or hermaphroditism. Further, in the higher animals these two organs exist in different individuals. This condition is called unisexuality. Thus there are several grades of sexuality. The sexual elements only may be separated, or in addition the sexual organs may be separated, or in addition there may be distinct sexual individuals. Any mode of reproduction not answering to this description is non-sexual. But non-sexual reproduction also is of different grades. The lowest is fission. A cell or a community of cells grows and divides itself into two. Each half, again, grows and divides, and so on ad infinitum. Next above this is budding. A spot on the external surface of an organism grows more rapidly than contiguous spots, and forms a tubercle which grows into a bud, assumes the form and structure of the parent, and finally separates. In the next grade the budding is internal, from a special organ simulating an ovary, though not a true ovary, as in aphides. Finally, in parthenogenesis we have a perfect ovary forming true ova and perfect embryo without fertilization or coöperation of the sperm-cell.
Now, my object, more specifically stated, is to show—1, that the highest form, viz., unisexuality, was developed out of bisexuality or hermaphroditism; 2, that bisexual reproduction was developed out of non-sexual reproduction; and, 3, that non-sexual reproduction is but an unessential modification of the ordinary process of growth.
4. Facts which furnish a Key to the Process of Derivation.—There are certain facts which throw light on each of these steps, but, as might be expected, the light is far clearer on the higher steps, because these were also the last taken.
(a.) Facts which bear on the Last Step, viz., the Derivation of Unisexuality from Bisexuality.—These facts are taken from both the vegetable and the animal kingdom, but especially the former. They are comprehended under the general term "cross-fertilization of bisexuals."
Plants.—It is a familiar fact that most plants are bisexual, i. e., have both ovary and spermary (anther-cell), in the same individual plant and in the same flower; and that nearly all such cases are capable of self-fertilization. But Mr. Darwin has shown that, although capable of self-fertilization, yet cross-fertilization—i. e., the fertilization of the ovules of one flower, or, still better, of the flowers of one plant by the pollen of another—produces more seeds, larger seeds, and stronger seedlings; in other words, produces better results. Now, it is a law which necessarily results from the principle of the survival of the fittest that Nature ever strives to secure better results. Therefore, she immediately sets to work to contrive methods of insuring cross-fertilization and preventing self-fertilization. The cross-fertilization is insured—1, by winds, aided by the lightness of the pollen; and, 2, by insects which carry the pollen from flower to flower. The beauty, the fragrance, and the honey of flowers are undoubtedly intended primarily to attract insects, and thus to insure cross-fertilization. But this alone is not sufficient. It is necessary also to prevent self-fertilization. This is done sometimes, as in orchids, by sticking together the pollen in masses by means of a gummy substance, so that it can not fly, and placing these masses entirely beyond the reach of the stigma, and sometimes by the maturation of the ovules and of the pollen at entirely different periods. In these cases the plant is wholly dependent upon insects for their fertilization, and we accordingly often find the most curious and ingenious contrivances in the structure of the flower to make sure that there be no failure in this respect. In other cases self-fertilization is still more effectually prevented by a separation of the sexes in different flowers (Monœcia), or in different plant individuals (Diœcia)—of course, winds and insects being still the carriers between the two sexes. This separation of the sexes was undoubtedly a gradual process. In bisexual plants, habitually cross-fertilized by winds or by insects, the one organ or the other became aborted until first only rudiments remained, and finally even these are lost and unisexuality is complete. These stages are sometimes detectable.
Animals.—In animals the process is probably similar. Many animals, such as oysters, polyps, etc., are bisexual and self-fertilizing. But even in these, cross-fertilization must be very common, if not the rule. These animals usually live together in great numbers; the sperm-particles are extremely light and abundant. These are therefore carried by waves and currents, so that the waters are full of them, and a promiscuous cross-fertilization is unavoidable. In fact, there can be no doubt that it is in order to insure this cross-fertilization that the sperm-particles are so light and abundant; and the final cause of this, again, is that cross-fertilization produces better results than self-fertilization. But if so, then Nature will take steps not only to insure cross-fertilization, but to prevent self-fertilization. This in animals as in plants can only be done in two ways, viz., either by so placing the two organs that self-fertilization is impossible, or else by separating them in different individuals. A curious example of the former method is found in snails. These animals are bisexuals—i. e., have both ovary and spermary perfect, but these are so placed that self-fertilization is impossible. They, therefore, mutually cross-fertilize. The latter method, of course, produces unisexuality, so universal in higher animals; but the process was probably the abortion, in habitually cross-fertilizing bisexuals, of one organ or the other in different individuals until unisexuality is established.
If, then, we compare plants and animals, we find the steps similar in the two kingdoms. Bisexual animals living together in numbers, and cross-fertilized by waves and currents, correspond to anemophilous flowers cross-fertilized by winds. Mutually fertilizing bisexuals like snails correspond to orchidaceous plants, except that the cross which is voluntary in the former is effected by insects in the latter. Finally, unisexuals in animals correspond to Diœcia in plants. In both kingdoms unisexuality is derived from bisexuality—in both because thus self-fertilization is prevented and cross-fertilization secured; and this, again, because thus a better result is secured in the offspring.
But the question has probably dwelt in your minds, "Why is it that cross-fertilization produces better results, i. e., stronger progeny, than self-fertilization?" There are probably two reasons: 1. The elaboration of both ovules and sperm in the same individual is wasteful of vital energy. The concentration of vital energy on one reproductive element secures that one product in a higher degree of perfection. Thus better sperm and better ovules combine to produce better ova and a stronger embryo. This is in accordance with the effect of differentiation of functions and organs of all kinds. 2. Again, in all cross-fertilization different individual characteristics are inherited by the common offspring. Now, among the many characteristics thus inherited from both sides in the offspring, there is a sort of struggle for life and a survival of only the fittest and strongest, and thus the offspring improves by the cross. Now, such cross is most completely secured by the separation of the sexes in different individuals—i. e., by unisexuality.
(b.) Facts which bear on the next Preceding Step, viz., the Derivation of Bisexuality from Asexuality.—This is doubtless the most obscure step; yet I believe some light is visible. Here is the greatest gap in the process; yet this gap may be largely filled.
Remember, then, that there is a striking correspondence between the embryonic or ontogenic series and the evolution or phylogenic series—that the former is a rapid recapitulation, as it were by memory, of the main points of the latter. The embryo repeats by a kind of organic memory the main point of its descent from primordial protoplasm. The lesser points, and especially the earliest points, often indeed drop out of memory, but usually the main points remain. Now, in all the higher animals, ontogeny is a continuous change without break, and completed in one generation. In many lower animals, however, there are apparent pauses, and sudden great changes in this process of ontogenic development. These are called metamorphoses. In insects, for example, there are two active conditions, the larva and the perfect insect, and a sort of second passive egg-stage between—the pupa. Here we have a semblance of, but not really, two generations. Of course, only the perfect insect reproduces. But in many still lower animals we find the metamorphoses occupying two or even more distinct generations. It follows, of course, that in these animals (contrary to what occurs in all higher animals) reproduction takes place both in the larval condition and in the perfect or mature condition. Now, the mode of reproduction in these two conditions is of wholly different kinds, the former being non-sexual and the latter sexual. A single example will suffice: The common medusæ or jelly-fishes, as you know, are unisexual—i. e., male and female. The fertilized females produce eggs which grow, not into medusæ, but into polyp-like animals which are the larval form. These polyps produce buds which open into flower-like bolls, then separate and swim away as male and female medusæ, which again produce eggs that spring up as polyp-like larvæ, etc. Here ontogenesis requires two generations to complete itself. In ontogenesis when both kinds of reproduction occur, the non-sexual (gemmation) precedes the sexual (ovulation). This fact strongly suggests, in fact renders almost certain, that the same is true in phylogenesis, or at least in the phylogenesis of this class.
But again: Aphids (plant-lice) also reproduce in the larval condition, and only reach maturity after many successive generations, sometimes as many as nine or ten. In spring these insects are hatched from eggs in a larval wingless condition. From an internal organ analogous to an ovary, but not a true ovary, these larvæ end another generation of larvæ like themselves, which in their turn, by internal budding, form a third generation, and so on until autumn, when the last generation develop into perfect winged insects, male and female. These last coöperate to produce eggs which hatch next spring, to commence another cycle of changes.
Here, then, we observe as before the lower form of reproduction in the larva, and the higher in the perfect insect. Here, again, we have non-sexual mode preceding the sexual mode in ontogenesis, suggesting a similar succession in phylogenesis. But in addition we observe here that the form of non-sexual reproduction very closely simulates sexual reproduction; for the budding is from an internal organ set apart for the purpose and very closely resembling a true ovary.
The next step in the chain of approximation is found in parthenogenesis or virgin generation. This consists in the formation, in a perfect female capable of sexual generation, of ovules which develop into embryos without the coöperation of the male element. In bees and wasps the ovules are sometimes fertilized and sometimes unfertilized. The fertilized eggs always produce females, the unfertilized always males. In this case the analogy to non-sexual reproduction is not close; because the female is, of course, the sex absolutely necessary to carry on the succession of generations, and it is this sex which it requires fertilization to produce. But in other cases, for example, in certain moths and in some phyllopod crustaceans, according to Siebold, the unfertilized eggs produce females and the fertilized males. In such cases, it is evident, a succession of females may be formed without the cooperation of the male; and thus we have continuous generation which is completely intermediate between sexual and non-sexual. It is sexual in that an embryo is developed from an ovule formed in a perfect ovary, it is non-sexual in that the coöperation of the male element is unnecessary even for an indefinite succession of generations.
On the other hand, the case of moths and phyllopod crustaceans approaches equally the case of aphids already mentioned—so much so, indeed, that the larval reproduction of these latter have often been classed under parthenogenesis. The difference is this: true parthenogenesis takes place in perfect females, capable of sexual union and of fertilization, possessing perfect ovaries and producing true ovules which develop into embryos without fertilization. The larval aphid, on the contrary, is not a perfect female, is not capable of sexual union nor of fertilization; its ovary-like organ is not a true ovary, does not produce true ovules which develop into embryos, but forms an embryo at once within, which then is born in an active state. Still the resemblance to parthenogenesis is undoubted, and together they almost wholly fill up the gap between the sexual and non-sexual modes of reproduction.
There is still another fact which must be brought forward to fill this gap. True sexual reproduction, as we have seen, consists essentially in the union or conjugation of two diverse cells (sperm and germ cell) to produce one cell (ovum). Now, in the lowest forms of sexual reproduction, among unicelled organisms, the conjugating cells are not perceptibly different; so that the element of diversity in the conjugating cells may be eliminated from the essential conditions of this mode. In parthenogenetic reproduction of female offspring, as in the case of moths and phyllopods, we have the other element, i, e., the necessity of two cells, eliminated; so that there remains nothing which is absolutely essential.
(c.) Facts which hear on the First Step, viz., Derivation of NonSexual Modes from Ordinary Processes of Growth.—The transition between the lowest form of non-sexual reproduction, viz., fission, and ordinary growth, is so obvious that it is hardly necessary to insist on it. A single cell divides itself into two; each half grows, and again divides itself into two, and so on. Now, if the cells cohere, we call it growth; if they separate, we call it reproduction. Again: a mass of cells grows by continued cell-multiplication, as above. Finally, the increasing mass or community becomes too large to be managed well from one center; it therefore divides itself into two masses or communities, each of which continues to grow as before. It is plain that a slight difference only in the degree of cohesion determines whether the same process be called growth or reproduction.
Thus we have shown the easy gradation, and therefore the probable derivation, of the highest mode of sexual reproduction—the unisexual—from the ordinary processes of growth, through the different grades of asexual and bisexual reproduction. The derivation of different modes of sexual reproduction from each other will not, I think, be questioned. Still clearer is the fact that non-sexual reproduction is but a modification of the ordinary process of growth. The only place where there is any gap is between the asexual and the sexual modes. Throughout growth and non-sexual modes of reproduction we have everywhere only cell-multiplication—everywhere we have division of one to form two: in sexual reproduction, on the other hand, we have the contrary process, viz., the union of two to form one. Yet this gap is certainly partly filled by the larval reproduction of aphids, by those cases of parthenogenesis in which unfertilized ovules produce females, and those cases of true sexual generation in which the conjugating cells are similar.
5. Outline of Probable History of the Process.—The gradual evolution of the higher forms of sexual reproduction probably took several different roads. There is little doubt that in some cases sexual reproduction in its simplest form was reached at a very early period. It is probable, for example, that in very early times unicelled organisms multiplying usually by fission (asexual) from time to time conjugated (sexual). The simple form of sexual reproduction thus reached was afterward perfected. But it is also probable, nay, judging from the transitional stages still in existence, almost certain, that in other cases sexuality was reached by a slower process and at a later period. It is this slower process which I now wish to trace in outline:
(a.) Fission.—In the lowest animals the individual cells which form their structure are almost wholly independent. The independent life of the cell is strong, the common life of the aggregate is feeble. By continued cell-multiplication, the aggregate, becoming too large to be held together by the common life, divides. Thus arises the lowest form of reproduction, viz., by fission. Many lower animals still practice this mode.
(b.) Budding on Any Part.—In the next step excess of growth occurs on any part indifferently, gives rise to a tubercle which grows into a bud, assumes the structure of the parent stem, and finally separates to become a new individual. This is higher than the last, because the original individual is not sacrificed, but only a part separated. Many larval medusæ and many polyps still practice this mode.
(c.) Budding on a Special Part.—In the last case the budding occurs in any part. In the next step a particular part is selected, and to it is assigned the function of forming buds which form new beings. Many larvæ of medusæ belong to this category; for they bud only on the mouth-disk. This is a higher form than the last, inasmuch as the assignment of a function to a particular place, or localization of a function, is an invariable step in evolution, and always attended with better results.
(d.) Special Budding Organ, internal.—The next step was probably the relegation of the function of producing buds to an internal organ, as being far safer and more certain of success, which organ thus becomes by position and function strongly analogous to an ovary. This is the case in larval aphids. The reproductive organ of these larvæ has been regarded by some as an ovary, by others as an internal budding organ. It is certainly not a true ovary, but rather perhaps an organ uniting the yet undifferentiated functions of ovary and spermary, an organ producing cells having the properties of both germ-cells and sperm-cells, and therefore capable of directly forming an embryo by cell-multiplication.
(e.) Differentiation of Sexual Elements.—The interior reproductive organ last described next forms two kinds of cells which by conjugation produce the ovum; the sexual elements are now differentiated, but not yet the sexual organs. It is not absolutely certain that this condition actually exists in any species now living; but it is probable that it does. According to Kleinenberg, the reproductive organ of the hydra produces both ovules and spermatozoids. In many mollusks and polyps the separation of the ovary and spermary is not yet made out. In some gasteropods the epithelial cells of the oviduct seem to become mother-cells, in which are produced spermatozoids. The separation here is only partial. Preceding the condition represented by the hydra, and connecting with the last (d), we ought to find one in which two similar cells elaborated by the same organ unite or conjugate to commence the new life—a condition in which the sexual elements are potentially but not visibly differentiated. This condition is realized, as far as we yet know, only in the conjugation of unicelled organisms.
(f.) Bisexuality.—The next step is of course the complete differentiation not only of the sexual elements, but also of the sexual organs. This is bisexuality or hermaphroditism, very common, as is well known, among lower animals and almost universal among plants.
(g.) Unisexuality.—The last step is the separation of the sexes in different individuals. This of course effectually prevents self-fertilization in both animals and plants. But cross-fertilization must be insured. This, as already seen, is done by winds and insects in the case of plants, and by waves and currents in some lower animals. These agents do not, however, insure fertilization in higher animals. Therefore, in them there is added sex-appetite and all associated feelings for that purpose.
(h.) After the separation of the sexes has been a sufficiently long time accomplished, the evidence in the ontogeny of former conditions is gradually obliterated—the memory of them is lost.
6. Differentiation of the Two Sexual Individuals.—We have now reached complete unisexuality—i. e., the separation of the sexes into different individuals, but not yet the very best results. Unisexuality is better than the orchid and snail method of mutual fertilization, only because the latter method is incapable of further differentiation, and therefore of any further improvement of results. In cases of mutual fertilization the individuals are all alike, except these small individual differences, which occur even in self-fertilizing bisexuals. But as soon as the sexes are separated into different individuals, then there is room for indefinite differentiation of the two sexual individuals. Now, as we go up the animal scale we find that such differentiation has indeed taken place, and that progressively. The sexual differences—i. e., the difference between male and female individuals of the same species—become greater and greater as we rise in the scale. They are also greater, we believe, in the higher as compared with the lower races of man, and in the cultivated classes as compared with the uncultivated classes. From this sexual difference springs sexual attractiveness, and from this lowest root, it is not too much to say, springs much if not all our noblest altruistic nature. For, as our physiological functions are primarily divisible into two great groups, viz., the nutritive and the reproductive, the one including all that assemblage of functions which conserve the individual life, the other all that assemblage of functions which conserve the continuous life of the species, so all our psychical functions are also primarily divisible into two groups, the egoistic and the altruistic—the one concerned only about the well-being of self, the other about the well-being of the race. These correspond each to each. Traced to its deepest physiological roots, the one in its last analysis is connected with the nutritive functions and the appetite for food, the other with the reproductive functions and the sexual appetite.
It seems to me not inappropriate to draw passing attention to the fact that that form of woman's rights which would assimilate as much as possible the two sexes is certainly in direct conflict with the law of evolution which we have been tracing. If founded in nature at all, we must seek for its justification in a higher law than that of animal evolution.
7. Crossing of Varieties.—Are there still further differentiations and still better results possible? Yes; by judicious crossing of varieties. Groups of individuals of both sexes, under the influence of differing environment, become different. This difference may be slight (slight varieties), or decided (strong varieties), or still more decided (races), or may become in time so great as to constitute distinct species. Now, it has been found that the indefinite interbreeding of individuals subject to identical conditions (close-breeding) produces weakness and degeneracy; and, on the contrary, the judicious crossing of slight varieties produces improved results. The reason is apparently this: Among all the qualities, good and bad, strong and weak, inherited from both sides by the offspring, there is a kind of struggle for life, and a survival of the best and the strongest qualities. It is probable that this improvement is more decided in the psychical than in the physical nature, and therefore is more conspicuous in man than in animals. Too close breeding—i. e., the interbreeding in isolated communities of individuals subject to identical conditions, and therefore with identical customs, habits, feelings, thoughts—tends to deteriorate the mind and character, even when the physique is unimpaired; tends to petrify the communal character and destroy that plasticity on which all progress depends.
Now, it is quite certain that within certain limits the improved results of crossing increase with the diversity of the crossing varieties. But mark, only within certain limits, beyond which they again decrease until deterioration is reached; and the deterioration increasing with the increasing divergence, when the crossing varieties reach a divergence represented by the term species, Nature practically forbids the ban. Thus, when species cross, there is either (a) no fertilization, and therefore no offspring; or (b), the offspring is an infertile hybrid, and therefore perishes in the next generation; or (c), if the offspring be fertile, the progeny is feeble, and perishes in the struggle for life in a few generations, or (d) is absorbed by crossing with the stronger parent species. If this were not so, species, in many cases at least, could not exist. Many species of oak or of pine grow in the same grove; the air is full of the pollen of many species; the conditions necessary for the crossing of different species must constantly occur, and yet the species remain distinct. The same is true of many hermaphrodite species of marine animals living in great numbers together; the water is full of the sperm of several species, and the conditions of cross-contact of sexual elements are constantly present, and yet species remain substantially distinct.
It is evident, therefore, that in close-breeding, and in the crossing of varieties of different degrees of divergence, there is, first, a less than average result, then an average, then better than an average, then this better result quickly reaches a maximum and again declines, crosses the line of average and becomes bad, and finally infinitely bad, or dies out. In the human species it is probable that the crossing of those varieties called national varieties, even strong national varieties, produces good results; but the crossing of varieties so divergent as those called primary races is probably bad—these approaching too nearly the nature of different species.
The general law of the effect of breeding may therefore be graphically represented by the following diagram, in which the absciss A B represents the level of average result, distance on this absciss from the middle point a represents the divergence of crossing varieties, and ordinates positive and negative represent the result of crossing, whether good or bad. Further, the middle point a represents no divergence or identical individuals, the distance b b individual differences, c c divergence constituting slight varieties, d d strong varieties, f f races, and g g species. By inspection of the figure it is seen that close-breeding (a) produces negative ordinates or bad results, then going from this
point the curve crosses the line of average at b b, then the ordinates become positive and reach maximum at d d, or strong varieties, then again crosses the line of average and becomes negative at f f, indicating the bad effect of crossing races, and finally becomes infinitely negative before it reaches g g, showing the practical infertility of crossing different species under natural conditions.
If I am right in this view, then the mixing of primary races is bad, and such mixed races, as weaker varieties in the struggle for life, must perish. There is one possibility which may save these races. Admitting the fact of deterioration as an immediate result of universal crossing of existing races, it is possible that by judicious crossing again of the slight varieties which must eventually arise in the mixed race, this common, inferior result may again be raised to a higher level. Thus, if the present higher races could consent to sacrifice their present superior position for several, perhaps many, generations, it is conceivable that the human race may be again raised, and possibly to a still higher plane. From a lower plane but broader base, it may be possible to build up again to a higher point than any yet reached. Or, to put it differently: the effect of true breeding is doubtless excellent in one direction, and for the perfecting of one or a few qualities, but it tends also to specialize, and therefore to petrify, and thus to prevent indefinite progress. Mixing, on the other hand, it produces a more plastic nature or better clay, a more generalized and therefore a more progressive form—for the line of true progress has ever been through generalized forms. Therefore it may be that, after the best results of true breeding have been attained in the production of the best varieties in several limited directions, then the general mixing of these perfected varieties will produce a generalized human type capable of more universal progress in all directions.