5. But still the question remains: Why are transitional forms rare in all cases, especially between species—so rare that they are eagerly sought and highly prized? I believe that the true reason of this is that the steps of evolution are not always uniform.
Nearly all evolutionists have assumed and even insisted on uniformity, as the opposite of catastrophism and of supernaturalism, and therefore as essential to the idea of evolution. They say that the constancy of the action of the forces of change necessitates the uniformity of the rate of change. But, in fact, this is not always nor even usually true. Causes or forces are constant, but phenomena everywhere and in every department of Nature are paroxysmal. The forces producing storms and lightning, and volcanoes and earthquakes, are or may be constant; yet the phenomena are in the highest degree paroxysmal. Wherever in nature we have a constant force and a strong resistance, we find more or less paroxysmal action. For this reason the wind blows in puffs, the friction of wind on water produces waves, water running in small pipes issues in pulses. The reason is obvious, as may be seen by the following examples: Suppose lifting forces within the earth are resisted by crust-rigidity. The forces accumulate uniformly until the resistance gives way, and suddenly we have an earthquake. Water running with great resistance in small pipes is checked, but soon accumulates additional force, which overcomes the resistance, only to be again checked, and so on, and therefore runs in pulses. Now, the course of evolution of the whole earth may be likened to such a current; there are forces of movement and forces of resistance—progressive forces and conservative forces. The progressive force is accumulative, the resisting force is constant. Thus, in all evolution or history, whether of the earth or of society, there are periods of comparative quiet, during which the forces of change are gathering strength, and periods of revolution or rapid change, during which these forces show themselves in conspicuous effects.
Now, that there have been such periods of rapid revolutionary change in the history of the earth, there can be no doubt. The history of the earth is marked by periods of comparative quiet, during which life was exceptionally abundant and prosperous, and change of organic forms slow and uniform—separated by periods of disturbance, revolution, rapid changes of physical geography and climate, and consequently of comparatively rapid and sweeping changes in organic forms. These form the division-lines between great eras of the earth’s history, and are always marked by extensive unconformity of the strata, showing the changes of physical geography above spoken of, and by apparently sudden and sweeping change in life-forms, showing the great changes of climate and other physical conditions. Unfortunately, in all cases of unconformity of strata, there is, of course, a break in the continuity of the record; and when the unconformity is very general a portion of the record may be irrecoverably lost. The consequence is, that there is an apparent break also in the continuity of life-forms. It looks, at first sight, like wholesale extermination of old and recreation of new forms. But undoubtedly the break in the continuity of life is apparent only, as is shown by the loss in the record. If we could recover the whole record, as indeed we sometimes do, we should find in all cases that there is no break in the continuity of evolution, but only more rapid rate of change at these times. But to this cause of rapid rate of progress—i. e., change of physical environment—we must add change of organic environment induced by the physical. We have already seen ([p. 179]) that extensive changes in physical geography and climate are always accompanied by wide migrations and dispersals of species, the mingling of faunas and floras, and the severer struggle for life, and the sweeping weeding-out of all but the fittest, and the change of these latter, making them still fitter. These two causes of rapid change, viz., change of climate and migrations, together with the loss of record, we believe completely account for those sweeping changes, not only of species but even of genera, families, and orders which characterize the passage from one great era to another.
But this does not yet explain the apparent discontinuity between consecutive species in the same locality in continuous, conformable strata, or the rarity of transitional forms when one species takes the place of another in an apparently continuous record. In such continuous deposits the successive faunas do indeed gradate insensibly into one another, but apparently as in contiguous geographical regions ([p. 200]) by substitution, not by transmutation. How shall we explain this?
On this point I throw out some suggestions: 1. In the modification of species, too, as well as in other progressive changes, we may imagine two forces operating, one progressive, the other conservative—the one external, the other internal. The external progressive force consists of all the factors of change already mentioned, the internal conservative is the law of heredity, of like producing like. A changing environment tends continually and increasingly to change of organisms, but change is resisted by heredity, which tends to adhere, within narrow limits, to the same form. But since the external force or tendency to change increases constantly—since the discord between the environment and the organism becomes ever greater, there must come a time when either the species is destroyed, or else the resistance of heredity gives way, and rapid change takes place. The alternative is presented to the species to transform or perish; and in one or perhaps in two or three generations we have an amount of change which, under other circumstances, might take a hundred generations to accomplish. These rapid changes are in fact exactly what in artificial varieties we call sports. We do not know all the conditions which determine sports in domestication, and still less what determines large and widely-divergent variations, and therefore rapid origin of many divergent species, in geological history. But one thing seems probable, viz., that, when a species begins to change, it continues to change easily and in many directions. When resistance gives way it takes some time, many generations, for heredity to gather force again. Hence, young species are plastic, fluent, because heredity, on any one point, has not yet accumulated. But as soon as a stable form is again reached, then, by accumulating a fund of heredity, the form tends to become more and more rigid, until often it becomes too rigid to yield to modifying influences, and therefore becomes extinct. By far the greater number of species do thus become extinct and leave no progeny, while the few more plastic forms are modified in several directions, and the number of forms may, after a little time, be undiminished or even increased.
2. As to the cause of rapid changes of form during revolutionary or critical periods in the earth’s history, Brooks has introduced an idea which is very suggestive, and deserves serious attention. We have above spoken of the progressive element as external. Brooks regards both elements as internal, and represented by the two sexes. The male represents the progressive, the female the conservative element. The one tends to divergent variation, the other to fixity of type by heredity. I think we will all admit that, as a general rule, in man (and probably all the higher animals) the male is more highly differentiated into many divergent forms—the female is more like the type-form of the species. In man, the male is certainly more diversified in form, in expression, and in character. If they have the keenest ear for musical pitch, they are also most often music-deaf; if they have the sharpest perception of color, they are also most often color-blind; if among them we find the brightest intellects, we also find the dullest and most stupid; if there are among them more geniuses, so, also, there are more cranks. The same is also, probably, true of other animals, in proportion to their grade of organization. The operation of these two equally necessary elements is well shown in every advancing society. The initiative of every movement, in all directions, good or bad, is determined by the male; the conservation of whatever balance of good there may be, seems to be mainly by the female. The male tries all things, the female holds fast that which is good. By the one society gains a little in each generation; by the other the gain is conserved and made a new point of departure. The one is ever building hastily a scaffolding and platform; the other ever consolidating into a permanent structure. Now, according to Brooks, what is true in the plane of social progress is true also in the lower plane of organic evolution. In sexual union, and in the resulting offspring, the sperm-cell is the element which tends to divergent variation, and the germ-cell to fixity of type, through heredity. In artificial breeding, then, we ought to make new varieties by proper use of the sire; we ought to preserve them true by proper management of the dam.
But, again, it is believed that in many lower animals, especially insects, the high-feeding of the mother, and consequent good condition of the ovum, tends to the production of female offspring. It seems almost certain that, in butterflies, the sex is not yet declared in the caterpillar stage. According to the careful experiments of Mrs. Treat,[37] if the caterpillars be well fed, they become female butterflies; but, if poorly fed, they make males. One purpose of this provision of Nature is, doubtless, to provide for the greater draught on the vitality of the female in reproduction.
Now for the application. In good times in the history of a species, when everything is prosperous, external conditions are favorable, and food is abundant, females are in excess, and individuals are greatly multiplied. Under these conditions, evolution would be slow and uniform. But in bad times in the history of a species, when external conditions were unfavorable, not only would there be excess of males, but these, through the influence of the changing environment, as well as through the dominance of the male element, would be more than usually varied in character. Among the strongly divergent varieties thus formed, the fittest—i. e., those most in accord with the changing environment—would survive and leave offspring partaking of their character. We have already repeatedly said that the severer pressure of a rapidly-changing environment determines correspondingly rapid changes in organic forms. It may do so in many ways; but, according to Brooks, one of the most important ways is by determining an excess of the male element.
In brief, then, the causes of rarity of transitional forms among fossils are—1. The change being, for the reasons given, comparatively rapid, the number of generations between consecutive species are few, perhaps only one. 2. Times of rapid change are also times of unfavorable conditions, and therefore the number of individuals in each generation is small, and all the smaller—in Brooks’s view—because of the fewness of females. When we remember that fossils are but a small fraction of the actual faunas and floras of the time, surely these two causes go far toward explaining the rarity of links between species. 3. Add to these the existence of periods of wide-spread changes in physical geography and climate, and consequent wide migrations and dispersals of species, and we sufficiently account for those sweeping changes in species, genera, families, and orders, which mark the limits of the great eras, and which are made still more abrupt, and apparently supernatural, by the loss of record at these times.[38]
Objection.—There is still one more objection which will be made. We have drawings of plants, animals, and men, by Egyptian artists, who lived at least three thousand years ago, and the species of the one and the races of the other are still the same. Still better, we have among the wrappings of Egyptian mummies the very plants themselves, leaves and flowers perfectly preserved, and even colors almost perfect. Yet the species are exactly the same as grow in Egypt to-day. If species are made by gradual transmutation, surely there ought to have been some change in three thousand years.