parative anatomy points out in the different diverging and ascending steps of the organic system, and which we call the systematic developmental series, corresponds with one portion of the palæontological developmental series; it deals with the anatomical result of the latter in the present; and is, at the same time, parallel with the individual developmental series; and this, again, is parallel with the palæontological series.

The varied differentiation, and the unequal degree of perfecting which comparative anatomy points out in the developmental series of the system, is chiefly determined by the ever-increasing variety of conditions of existence to which the different groups adapt themselves in the struggle for life, and by the different degrees of rapidity and completeness with which this adaptation has been effected. Conservative groups which have retained their inherited peculiarities most tenaciously remain, in consequence, at the lowest and rudest stage of development. Those groups progressing most rapidly and variously, and which have adapted themselves to changed conditions of existence most readily, have attained the highest degree of perfection. The further the organic world developed in the course of the earth's history, the greater must the gap between the lower conservative and the higher progressive groups have become, as in fact may be seen too in the history of nations. In this way also is explained the historical fact, that the most perfect animal and vegetable groups have developed themselves in a comparatively short time to a considerable height, while the lowest or most conservative groups have remained stationary throughout all ages in their original simple stage, or have progressed, but very slowly and gradually.

THE THREE SERIES PARALLEL.

361

The series of man's progenitors clearly shows this state of things. The sharks of the present day are still very like the primary fish, which are among the most ancient vertebrate progenitors of man, and the lowest amphibians of the present day (the gilled salamanders and salamanders) are very like the amphibians which first developed themselves out of fishes. So, too, the later ancestors of man, the Monotremata and Marsupials, the most ancient mammals, are at the same time the most imperfect animals of the class which still exist. The laws of inheritance and adaptation known to us are completely sufficient to explain this exceedingly important and interesting phenomenon, which may be briefly designated as the parallelism of individual, of palæontological, and of systematic development, and of their respective progress and differentiation. No opponent of the Theory of Descent has been able to give an explanation of this extremely wonderful fact, whereas it is perfectly explained, according to the Theory of Descent, by the laws of Inheritance and Adaptation.

If we examine this parallelism of the three organic series of development more accurately, we have to add the following special distinctions. Ontogeny, or the history of the individual development of every organism (embryology and metamorphology), presents us with a simple unbranching or graduated chain of forms; and so it is with that portion of phylogeny which comprises the paleontological history of development of the direct ancestors of every individual organism. But the whole of phylogeny—which meets us in the natural system of every organic tribe or phylum, and which is concerned with the investigation of the palæontological development of all the branches of

this tribe-forms a branching or tree-shaped developmental series, a veritable pedigree. If we examine and compare the branches of this pedigree, and place them together according to the degree of their differentiation and perfection, we obtain the tree-shaped, branching, systematic developmental series of comparative anatomy. Strictly speaking, therefore, the latter is parallel only to a portion of the whole of phylogeny, and consequently only partially parallel to ontogeny; for ontogeny itself is parallel only to a portion of phylogeny.

Of late years it has been a much-disputed point which of the three great series of development is of most importance to transformism and for our knowledge of the primary relationships. This dispute is superfluous; for, as a rule, all three are of equal value; in individual cases, however, the phylogenetic investigator will have to examine every special case critically to ascertain whether he is to set greater value on the facts of palæontology, of ontogeny, or of comparative anatomy.

All the phenomena of organic development above discussed, especially the threefold genealogical parallelism, and the laws of differentiation and progress, which are evident in each of these three series of organic development, are exceedingly important proofs of the truth of the Theory of Descent. For by it alone can they be explained, whereas its opponents cannot even offer a shadow of an explanation of them. Without the Doctrine of Filiation, the fact of organic development in general cannot be understood. We should, therefore, for this reason alone, be forced to accept Lamarck's Theory of Descent, even if we did not possess Darwin's Theory of Selection.

CHAPTER XIV.

MIGRATION AND DISTRIBUTION OF ORGANISMS. CHOROLOGY AND THE ICE PERIOD OF THE EARTH.

Chorological Facts and Causes.-Origin of most Species in one Single Locality: "Centres of Creation."-Distribution by Migration.-Active and Passive Migrations of Animals and Plants.-Flying Animals.Analogies between Birds and Insects.-Bats.-Means of Transport.— Transport of Germs by Water and by Wind.-Continual Change of the Area of Distribution by Elevations and Depressions of the Ground.Chorological Importance of Geological Processes.-Influence of the Change of Climate.-Ice or Glacial Period.-Its Importance to Chorology.-Importance of Migrations for the Origin of New Species. -Isolation of Colonists.-Wagner's Law of Migration.-Connection between the Theory of Migration and the Theory of Selection.-Agreement of its Results with the Theory of Descent.

As I have repeatedly said, but cannot too much emphasize, the actual value and invincible strength of the Theory of Descent does not lie in its explaining this or that single phenomenon, but in the fact that it explains all biological phenomena, that it makes all botanical and zoological series of phenomena intelligible in their relations to one another. Hence every thoughtful investigator is the more firmly and deeply convinced of its truth the more he advances from single biological observations to a general view of the whole domain of animal and vegetable life.

Let us now, starting from this comprehensive point of view, survey a biological domain, the varied and complicated phenomena of which may be explained with remarkable simplicity and clearness by the theory of descent. I mean Chorology, or the theory of the local distribution of organisms over the surface of the earth. By this I do not only mean the geographical distribution of animal and vegetable species over the different parts and provinces of the earth, over continents and islands, seas, and rivers; but also their topographical distribution in a vertical direction, their ascending to the heights of mountains, and their descending into the depths of the ocean.

The strange chorological series of phenomena which show the horizontal distribution of organisms over parts of the earth, and their vertical distribution in heights and depths, have long since excited general interest. In recent times Alexander Humboldt 39 and Frederick Schouw have especially discussed the geography of plants, and Berghaus, Schmarda, and Wallace the geography of animals, on a large scale. But although these and several other naturalists have in many ways increased our knowledge of the distribution of animal and vegetable forms, and laid open to us a new domain of science, full of wonderful and interesting phenomena, yet Chorology as a whole remained, as far as their labours were concerned, only a desultory knowledge of a mass of individual facts. It could not be called a science as long as the causes for the explanation of these facts were wanting. These causes were first disclosed by the theory of selection and its doctrine of the migrations of animal and vegetable species, and it is only since Darwin that we have been able to speak of an independent science