III.

THE ELEMENTS OF ORGANIC EVOLUTION.

ALL the laws of life, whatever their nature, are valid throughout the organic world. They control the life processes of man, those of the lower animals, and those of "our brother organisms, the plants." They extend to each in its degree. The fact that the laws of heredity, for example, extend unchanged in essence from one extreme of organic life to another is most vital to our understanding of the nature of life. For such homology as this, for any fact of homology whatsoever, we have found but one cause, the influence of common descent.

There are many elements or factors which enter into the processes of organic evolution, and they stand in varied relations to one another. It is not possible to make a classification of them in which there shall not be inequality and overlapping of elements. For the purpose of our present discussion we may group these forces and factors under eight principal heads.

I. Heredity. This is the "law of persistence in a series of organisms." Throughout Nature each creature tends to reproduce its own qualities and those of its ancestors. "Like begets like." Creatures resemble their ancestors. The germ cell specialized for purposes of reproduction is capable in its development "of repeating the whole with the precision of a work of art." Heredity is the great conservative force of evolution.

Its influence is shown in the persistence of type, in the existence of broad homologies among living forms, in the possibility of natural systems of classification in any group, in the retention of vestigial organs, in the early development and subsequent obliteration of outworn structures once useful to the race or type.

The physical basis of heredity has been in recent years the subject of many elaborate investigations. The complete homology of the germ cell with the one-celled animals, or protozoa, is now generally recognised, and there is large reason to believe that in the bands and loops of the nucleus of the germ cell is found the visible vehicle by which hereditary tendencies are transmitted.

II. Irritability.-All living beings are affected by their environment. Living matter must always respond in some degree to every external stimulus. All living beings are moved by or react from every phase of their surroundings. The nervous system and its associated sense organs are directly related to the conditions of life. They are concessions made to the environment. The power of motion, whatever it may be, requires the guidance obtained from the impressions made by external things. In all animals this knowledge, whatever its degree of completeness, tends to work itself out in action. In plants the same thing is in some degree. The essential difference is that, having no power of locomotion, the plant is without a general sensorium. The parts that move-growing rootlets, tips of branches, and the like-have sensibility and power of motion in the same series of cells. The animal, a colony of cells which move as a whole, has a specialized nervous system which guides the whole.

true.

As a rule, the environment does not act directly on the individual. Its influence is felt chiefly in modifying its action, in increasing, diminishing, or changing its.

efforts. The effects of environment are practically recognised in processes of education, of agriculture, the care and nurture of men and of horses and trees and wheat. Evil surroundings produce evil effects. Easy surroundings, reducing the stimulus to effort, tend to produce organic degeneration. In larger ways response to environment produces a long series of "concessions." A character or condition in itself of the nature of a response to outside stimulus may be called a concession. Among such concessions are the skin, the eyes, the brain, the sense of pain, in fact, in the ultimate analysis, every organ and every function of the body. For without environment all these would be unnecessary. Their existence would be inconceivable.

The fitness by which organisms have been perpetuated is simply obedience or adaptation. Those which survive are fitted to the conditions of life. In other words, they are obedient to these conditions. Hence we may define the process as one of the survival of the obedient. The force which commands obedience is that of the environment, and the obedience demanded is that of such a reaction or relation to this environment as will not obstruct the processes of life.

Concessions of

life.

Every form or phase of obedience shows itself as adaptation. Every adaptation is a concession to the actual environment on the one hand, to the laws of life on the other. The function of the eye, for example, is to give information as to the nature of objects more or less remote from the organism. The purpose of giving this knowledge is to enable the organism to act upon it. To be able to act demands that the action must be safe. If the creature could not act, it would have no need for such knowledge. If its acts were not in accord with knowledge, the knowledge would be useless. If there

were no break in the uniformity of the environment, there would be no need of such knowledge. If there were no variation in lights and shadows, the eye would be powerless to bring information. The senses deal with changes or breaks in reality rather than with realities themselves. Because, in action, the organism must be obedient to the demands of its environment, it is the function of the eye to make known these demands. The existence of the eye is therefore a concession to the environment. A concession of like nature is the brain itself, of which the eye and the sense organs in general may be considered as prolongations. These appendages of the brain carry to it truth of varying kind or degree. This truth as to external nature furnishes the basis of that obedience which in the animal expresses itself in action.

The respiratory apparatus is an adaptation for the purpose of purifying the blood from the waste produced in the processes of life. It is a concession on the one hand to the demands of life in cell and tissue, and on the other hand to the nature of the surrounding medium. A change in the atmosphere would demand a corresponding change in the organs of breathing. If such a concession were impossible, the species in question would become extinct, as its individuals would perish. If the concessions necessary to continued existence should involve changes in other organs, the process of the survival of the obedient would in time produce these changes.

If there were no surrounding medium there would be no organ of respiration. If there were no light there would be no organ of vision. If there were no sound there would be no ear. If there were no motion there would be no need for knowledge, and therefore no sensation. If there were no power of locomotion there

would be no sensorium. If there were no environment there would be no concessions to it. Without concession there would be no specialization of functions or organs. Without variation in environment there could be no choice in action. The concessions to the environment constitute, therefore, practically the whole structure. of any animal and the whole of the functions of its life. It is in the response to environment, the concession, the adaptation, the specialization, that the progress of life consists. It is in characters thus produced that man and the higher animals differ from the protozoa. Even the protozoan has its concessions. The phenomenon of growth causes the substance of the one-celled animal to increase faster than its absorptive power. The waste of the body varies as the substance—that is, as the cube of the diameter of the creature. The absorptive power of its surface must increase as the square of the diameter —that is, as the surface. Hence, a one-celled animal passing a given small size must either starve to death or else make some concession to its surroundings. This concession is reproduction-the one-celled creature must split into two animals. This increases the digestive power, with no increase of substance. Even the presence of skin on a protozoan is a concession to its surroundings. That a given protozoan is developed with an outside covering shows that natural selection has been long at work on its ancestry in preparing such a concession to external demands.

A creature which had known no environment and which had inherited no concession would be formless and structureless. It could be little if anything more than an organic molecule, or at the most a nebulous mist of organic molecules without parts or form or function. We know no such nebulous life as this. All the animals and plants on the records of science show traces of