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.—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 selection. 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. (Gen. Morph. ii. 286.)
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 and Schmarda 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 the works of Darwin and Wallace that we have been able to speak of an independent science of Chorology.
If all the phenomena of the geographical and topographical distribution of organisms are examined by themselves, without considering the gradual development of species, and if at the same time, following the customary superstition, the individual species of animals and plants are considered as forms independently created and independent of one another, then there remains nothing for us to do but to gaze at those phenomena as a confused collection of incomprehensible and inexplicable miracles. But as soon as we leave this low stand-point, and rise to the height of the theory of development, by means of the supposition of a blood-relationship between the different species, then all at once a clear light falls upon this strange series of miracles, and we see that all chorological facts can be understood quite simply and clearly by the supposition of a common descent of the species, and their passive and active migrations.
The most important principle from which we must start in chorology, and of the truth of which we are convinced by due examination of the theory of selection, is that, as a rule, every animal and vegetable species has arisen only once in the course of time and only in one place on the earth—its so-called “centre of creation”—by natural selection. I share this opinion of Darwin’s unconditionally, in respect to the great majority of higher and perfect organisms, and in respect to most animals and plants in which the division of labour, or differentiation of the cells and organs of which they are composed, has attained a certain stage. For it is quite incredible, or could at best only be an exceedingly rare accident, that all the manifold and complicated circumstances—all the different conditions of the struggle for life, which influence the origin of a new species by natural selection—should have worked together in exactly the same agreement and combination more than once in the earth’s history, or should have been active at the same time at several different points of the earth’s surface.
On the other hand, I consider it to be very probable that certain exceedingly imperfect organisms of the simplest structure, forms of species of an exceedingly indifferent nature, as, for example, many single-celled Protista, but especially the Monera, the simplest of them all, should have several times or simultaneously arisen in their specific form in several parts of the earth. For the few and very simple conditions by which their specific form was changed in the struggle for life may surely have often been repeated, in the course of time, independently in different parts of the earth. Further, those higher specific forms also, which have not arisen by natural selection, but by hybridism (the previously-mentioned hybrid species, pp. 147 and 275), may have repeatedly arisen anew in different localities. As, however, this proportionately small number of organisms does not especially interest us here, we may, in respect of chorology, leave them alone, and need only take into consideration the distribution of the great majority of animal and vegetable species in regard to which the single origin of every species in a single locality, in its so-called “central point of creation,” can be considered as tolerably certain.
Every animal and vegetable species from the beginning of its existence has possessed the tendency to spread beyond the limited locality of its origin, beyond the boundary of its “centre of creation,” or, in other words, beyond its primæval home, or its natal place. This is a necessary consequence of the relations of population and over-population (pp. 161 and 256). The more an animal or vegetable species increases, the less is its limited natal place sufficient for its sustenance, and the fiercer the struggle for life; the more rapid the over-population of the natal spot, the more it leads to emigration. These migrations are common to all organisms, and are the real cause of the wide distribution of the different species of organisms over the earth’s surface. Just as men leave over-crowded states, so all animals and plants migrate from their over-crowded primæval homes.
Many distinguished naturalists, especially Lyell[(11)] and Schleiden, have before this repeatedly drawn attention to the great importance of these very interesting migrations of organisms. The means of transport by which they are effected are extremely varied. Darwin has discussed these most excellently in the eleventh and twelfth chapters of his work, which are exclusively devoted to “geographical distribution.” The means of transport are partly active, partly passive; that is to say, the organism effects its migration partly by free locomotion due to its own activity, and partly by the movements of other natural bodies in which it has no active share.
It is self-evident that active migrations play the chief part in animals able to move freely. The more freely an animal’s organization permits it to all move in directions, the more easily the animal species can migrate, and the more rapidly it will spread over the earth. Flying animals are of course most favoured in this respect, among vertebrate animals especially birds, and among articulated animals, insects. These two classes, as soon as they came into existence, can have more easily spread over the whole earth than any other animal, and this fact partly explains the extraordinary uniformity of structure which characterizes these two great classes of animals. For, although they contain an exceedingly large number of different species, and although the insect class alone is said to possess more different species than all other classes of animals together, yet all the innumerable species of insects, and in like manner, also, the different species of birds, agree most strikingly in all essential peculiarities of their organization. Hence, in the class of insects, as well as in that of birds, we can distinguish only a very small number of large natural groups or orders, and these few orders differ but very little from one another in their internal structure. The orders of birds with their numerous species are not nearly as distinct from one another as the orders of the mammalian class, containing much fewer species; and the orders of insects, which are extremely rich in genera and species, resemble one another much more closely in their internal structure than do the much smaller orders of the crab class. The general parallelism between birds and insects is also very interesting in relation to systematic zoology; and the great importance of their richness in forms, for scientific morphology, lies in the fact that they show us how, within the narrowest anatomical sphere, and without profound changes of the essential internal organization, the greatest variety in external bodily forms can be attained. The reason of this is evidently their flying mode of life and their free locomotion. In consequence of this birds, as well as insects, have spread very rapidly over the whole surface of the earth, have settled in all possible localities inaccessible to other animals, and variously modified their specific form by superficial adaptation to particular local relations.