chapter xvi

.)

It is self-evident, therefore, that Wagner’s theory is quite inapplicable to all these non-sexual organisms. Moreover, the same applies to all those hermaphrodites in which every individual possesses both male and female organs and is capable of self-fructification. This is the case, for instance, in the Flat-worms, flukes, and tape-worms, further in the important Sack-worms (Tunicates), the invertebrate relatives of the vertebrate animals, and in very many other organisms of different groups. Many of these species have arisen by natural selection, without a “crossing” of the originating species with its primary form having been possible.

As I have already shown in the eighth chapter, the origin of the two sexes, and consequently sexual propagation in general, must be considered as a process which began only in later periods of the organic history of the earth, being the result of differentiation or _division of labour_. The most ancient terrestrial organisms can have propagated themselves only in the simplest non-sexual manner. Even now all Protista, as well as all the countless forms of cells, which constitute the body of higher organisms, multiply themselves only by non-sexual generation. And yet there arise here “new species” by differentiation in consequence of natural selection.

But even if we were to take into consideration the animal and vegetable species with separate sexes, in this case too we should have to oppose Wagner’s chief proposition, that “the _migration_ of organisms and their formation of colonies is the _necessary condition of natural selection_.” August Weismann, in his treatise on the “Influence of Isolation upon the Formation of Species,”(24) has already sufficiently refuted that proposition, and has shown that even in one and the same district one bi-sexual species may divide itself into several species by natural selection. In relation to this question, I must again call to mind the great influence which _division of labour, or differentiation_, possesses, being one of the necessary results of natural selection. All the different kinds of cells constituting the body of the higher organisms, the nerve cells, muscle cells, gland cells, etc., all these “good species,” these “bonæ species” of elementary organisms, have arisen solely by division of labour, in consequence of natural selection, although they not only never were locally isolated, but ever since their origin have always existed in the closest local relations one with another. Now, the same reasoning that applies to these elementary organisms, or “individuals of the first order,” applies also to the many-celled organisms of a higher order which only at a later date have arisen as “good species” from among their fellows.

We are therefore of the same opinion as Darwin and Wallace, that the migration of organisms and their isolation in their new home is a very advantageous condition for the origin of new species; but we cannot admit, as Wagner asserts, that it is a _necessary_ condition, and that without it no species can arise. Wagner sets up this opinion, “that migration is a necessary condition for natural selection,” as a special “_law of migration_”; but we consider it sufficiently refuted by the above-mentioned facts. We have, moreover, already pointed out that in reality the origin of new species by natural selection is a _mathematical and logical necessity_ which, without anything else, follows from the simple combination of three great facts. These three fundamental facts are—the Struggle for Life, the Adaptability, and the Hereditivity of organisms.

We cannot here enter into detail concerning the numerous interesting phenomena furnished by the geographical and topographical distribution of organic species, which are all wonderfully explained by the theory of selection and migration. For these I refer to the writings of Darwin,(1) Wallace,(36) and Moritz Wagner,(40) in which the important doctrine of the _limits of distribution_—seas, rivers, and mountains—is excellently discussed and illustrated by numerous examples. Only three other phenomena must be mentioned here on account of their special importance. First, the close relation of forms, that is, the striking “family likeness” existing between the characteristic local forms of every part of the globe, and their extinct fossil ancestors in the same part of the globe; secondly, the no less striking “family likeness” between the inhabitants of island groups and those of the neighbouring continent from which the islands were peopled; lastly and thirdly, the peculiar character presented in general by the flora and fauna of islands taken as a whole.

All these chorological facts given by Darwin, Wallace, and Wagner—especially the remarkable phenomena of the limited local fauna and flora, the relations of insular to continental inhabitants, the wide distribution of the so-called “cosmopolitan species,” the close relationship of the local species of the present day with the extinct species of the same limited territory, the demonstrable radiation of every species from a single central point of creation—all these, and all other phenomena furnished to us by the geographical and the topographical distribution of organisms, are explained in a simple and thorough manner by the theory of selection and migration, while without it they are simply incomprehensible. Consequently, in the whole of this series of phenomena we find a new and weighty proof of the truth of the Theory of Descent.

[1] The world is perfect save where Man Comes in with his strife.

[2] Archebiosis (Bastian), Abiogenesis (Huxley).

[3] Alle Glieder bilden sich aus nach ew’gen Gesetzen, Und die seltenste Form bewahrt im Geheimniss das Urbild. Also bestimmt die Gestalt die Lebensweise des Thieres. Und die Weise zu leben, sie wirkt auf alle Gestalten Mächtig zurück. So zeiget sich fest die geordnete Bildung, Welche zum Wechsel sich neigt durch äusserlich wirkende Wesen.

[4] “Einstweilen bis den Bau der Welt Philosophie zusammenhält, Erhält sich ihr Getriebe Durch Hunger und durch Liebe.”

[5] “Allgemeine Naturgeschichte und Theorie des Himmels.”

[6] We must wait for fuller information on the subject of Bathybius, at the hands of the naturalists of the _Challenger_ expedition, before accepting it finally as a distinct organism.—_Editor._

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