If we follow the historical development of the vegetable kingdom we shall find the same law operative there. Of plants there existed at first only the lowest and most imperfect classes, the Algæ or tangles. Later there followed the group of Ferns or Filicinæ (ferns, pole-reeds, scale-plants, etc.). But as yet there existed no flowering plants, or Phanerogama. These originated later with the Gymnosperms (firs and cycads), whose whole structure stands far below that of the other flowering plants (Angiosperms), and forms the transition from the group of fern-like plants to the Angiosperms. These latter developed at a still later date, and among them there were at first only flowering plants without corolla (Monocotyledons and Monochlamyds); only later were there flowering plants with a corolla (Dichlamyds). Finally, again, among these the lower polypetalous plants preceded the higher gamopetalous plants. The whole series thus constitutes an irrefutable proof of the great law of progressive development.
Now, if we ask what is the cause of this fact, we again, just as in the case of differentiation, come back to natural selection in the struggle for life. If once more we consider the whole process of natural selection, how it operates through the complicated interaction of the different laws of Inheritance and Adaptation, we shall recognize not only divergence of character, but also the perfecting of structure to be the direct and necessary result of it. We can trace the same thing in the history of the human race. Here, too, it is natural and necessary that the progressive division of labour constantly furthers mankind, and urges every individual branch of human activity into new discoveries and improvements. This progress itself universally depends on differentiation, and is consequently, like it, a direct result of natural selection in the struggle for life.
CHAPTER XII.
LAWS OF DEVELOPMENT OF ORGANIC TRIBES AND OF INDIVIDUALS. PHYLOGENY AND ONTOGENY.
Laws of the Development of Mankind: Differentiation and Perfecting.—Mechanical Cause of these two Fundamental Laws.—Progress without Differentiation, and Differentiation without Progress.—Origin of Rudimentary Organs by Non-use and Discontinuance of Habit.—Ontogenesis, or Individual Development of Organisms.—Its General Importance.—Ontogeny, or the Individual History of Development of Vertebrate Animals, including Man.—The Fructification of the Egg.—Formation of the three Germ Layers.—History of the Development of the Central Nervous System, of the Extremities, of the Branchial Arches, and of the Tail of Vertebrate Animals.—Causal Connection and Parallelism of Ontogenesis and Phylogenesis, that is of the Development of Individuals and Tribes.—Causal Connection of the Parallelism of Phylogenesis and of Systematic Development.—Parallelism of the three Organic Series of Development.
If man wishes to understand his position in nature, and to comprehend as natural facts his relations to the phenomena of the world cognisable by him, it is absolutely necessary that he should compare human with extra-human phenomena, and, above all, with animal phenomena. We have already seen that the exceedingly important physiological laws of Inheritance and Adaptation apply to the human organism in the same manner as to the animal and vegetable kingdoms, and in both cases interact with one another. Consequently, natural selection in the struggle for life acts so as to transform human society, just as it modifies animals and plants, and in both cases constantly produces new forms. The comparison of the phenomena of human and animal transformation is especially interesting in connection with the laws of divergence and progress, the two fundamental laws which, at the end of the last chapter, we proved to be direct and necessary consequences of natural selection in the struggle for life.
A comparative survey of the history of nations, or what is called “universal history,” will readily yield to us, as the first and most general result, evidence of a continually increasing variety of human activities, both in the life of individuals and in that of families and states. This differentiation or separation, this constantly increasing divergence of human character and the form of human life, is caused by the ever advancing and more complete division of labour among individuals. While the most ancient and lowest stages of human civilization show us throughout the same rude and simple conditions, we see in every succeeding period of history, among different nations, a greater variety of customs, practices, and institutions. The increasing division of labour necessitates an increasing variety of forms corresponding to it. This is expressed even in the formation of the human face. Among the lowest tribes of nations, most of the individuals resemble one another so much that European travellers often cannot distinguish them at all. With increasing civilization the physiognomy of individuals becomes differentiated, and finally, among the most highly civilized nations, the English and Germans, the divergence in the characters of the face is so great that we very rarely mistake one face for another.
The second great fundamental law which is obvious in the history of nations is the great law of progress or perfecting. Taken as a whole, the history of man is the history of his progressive development. It is true that everywhere and at all times we may notice individual retrogressions, or observe that crooked roads towards progress have been taken, which lead only towards one-sided and external perfecting, and thus deviate more and more from the higher goal of internal and enduring perfecting. However, on the whole, the movement of development of all mankind is and remains a progressive one, inasmuch as man continually removes himself further from his ape-like ancestors, and continually approaches nearer to his own ideal.