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 EVOLUTION

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EVOLUTION

criterion of the elementary species is the experimen- tally proved constancy of the features (it is quite im- material how small they may be) during a series of generations.

How are we to regard these opinions? Before an- swering this question we must strongly emphasize the fact that the biological idea of species has nothing whatever in common with the Scriptural conception or with that of Scholastic philosophy. The Mosaic story of Creation signifies nothing more than this, that ultimately all organisms owe their existence to the Creator of the world. The concrete hoiv has nothing to do with the proposition of faith regarding creation. The enumeration of certain popular groups of organ- isms, such as fruit-trees, draft-animals, and the like, could have no other design than to manifest to the simplest as well as to the most cultivated mind the action of the Creator of all things; at least, there can be no question of a scientific conception of genera and species. The biological concept of species is likewise removed from the philosophical concept wiiich desig- nates either the metaphysical or the physical species. The former is identical with the inlegra essentia (Urra- burii) — "integral essence" — of a being; the latter is founded on the essence {fundatur in essentid — T. Pesch), and is to be recognized by some attribute {gradus alicujus perfedionis) which remains constant and unchangeable in every individual of every genera- tion and so appears to be necessarily connected with the most intimate essence of the organism (necessario cum rei naturd connecti — Haan). The concept, there- fore, of species according to Holy Scripture, Philoso- phy, and Science, is by no means a synonymous one for the natural units of the organic world. And particu- larly, the first chapter of Genesis should not be brought into connexion with Linnseus's "Systema naturaj ".

As far as the biological concept of species is con- cerneil there is not up to the present time any decisive criterion by which we may determine in practice whether a given group of organisms constitute a par- ticular species or not. Genuine species are differenti- atei.1 from one another by the fact of their possessing some important morphological difference which re- mains constant dm-ing a series of generations without the production of any intermediate form. If the dif- ferences are of less importance, but constant, we speak of sub-species (elementary species, Jordan species), while intermediate forms and all deviations which are not strictly constant are set down as varieties. Are such distinctions and criteria acceptable? Expres- sions such as "considerable", "essential", "more or less considerable " signify relative propositions. Hence it follows that the morphological determination of species depends to a great extent on the sul)jective es- timate of the naturalist and on his intimate knowledge of the geographical distribution and habits of the or- ganism concerned. In fact, the force of the term species differs greatly in the different classes of organ- isms. On this account the fact that species do not cross-breed, or at least that after a cross they do not produce fertile descendants, was added as an au.xiliary criterion. This criterion, however, is an impractica- ble one in the case of palajontological species, and in the plant world in particular has many exceptions. In botany, therefore, the auxiliary criterion has been lim- ited in the sense that within the species itself the fer- tility always maintains the same general level, while by the crossing of tlifferent species it diminishes very materially — proposit ions which do not admit of conver- sion and in their generalization can scarcely be called correct. Consequently, it would almost appear that Darwin was right when he said that the idea of species was "undefinable". Still, it is not to be denied that tlirre are in nature definite and often important grada- tions and gaps by which the " gooil species", in contra- distinction to the "bad species", are separated from

one another. The same is also proved by the modern "mutation theories" which, on account of uncon- nected differences, admit a development of species by jumps.

The Darwinian principle of indefinite variability is contrary to facts, which in general show that, both in living nature and in geological strata, there exist types sharply tliscriminatetl from one another. How- ever, it is quite impossible to say how many types compose the organic world. It will be the task of future research to determine the affinity which exists between the various groups of organisms, beginning with the lower limit of similar sub-species and ascend- ing to the highest forms whose common ancestry can be proved. These highest forms, which per se have nothing in common with the Linnaean species or gen- era, or with any other systematic groups, are the true units of nature ; for they are composed of those organ- isms only which are related among themselves without being connected with the rest by common descent. We may, if we wish, identify these highest units with Wasmann's "natural species", or primeval ancestral forms, but, according to our opinion, neither the Lin- nsean species nor any other of the so-called systematic groups can be considered as the natural subdivisions of it. The Linnaean species are indeed indispensable for an intelligible classification of organisms, but they are not suitable for the solution of the problem of de- velopment. In concluding this section we may add that the best example of a natural species, and one ratified by revelation, is the species Man, which, by reason of its wide range of variation and the relative constancy of its races, may offer many a happy point of comparison for defining the limits of the species in the vegetable and animal kingdoms.

In the following sections we shall see that there can- not be any doubt as to the evolution of species, if by species we understand such groups of organisms as are generally styled by botanists and zoologists syste- matic, or Linnaean, species. But if by the term species we are to understand groups of organisms whose range of variability would correspond to that of " the human species", then we believe that up to the present day there are no clear facts in favour of specific evolution. In particular, it will be seen that thus far there is no evidence of fact as to an ascending development of organic forms, though we do not deny the possibility of it provided an innate power of development be as- sumed, which operates teleologically.

III. V.4.Ri.\TioN AND Experimental Facts relat- ing TO THE Evolution of Species. — By variation we generally understand three groups of phenomena: (1) individual differences; (2) single variations; (3) forms produced by crossing and Mendelian segrega- tion. The question is, what influence these variations actually have on the formation of species.

(1) Individual Differences. Individual differences include all fluctuating inequalities of an individual and of its organs — e. g., the size of the leaves of a tree, the percentage of sugar contained in the beet, and even more important morphological and physiological features. These differences may be quantitative (according to size and weight), meristic (as to num- bers), and individually quantitative (e. g., the moun- tain and valley forms of a plant). They are generally recognized from the fact that they oscillate around a certain mean, from which they deviate in inverse pro- portion to their frequency, a rule which primarily per- tains only to quantitative differences. According to Darwinians, useful indiviilual differences can be in- creased indefinitely liy .selection and may finally be- come independent of it. In this manner new species would result: Darwin himself sometimes considered single variations as of greater importance. The same view is strongly defended by modern evolutionists, who defend, at the same time, a direct influence of en- vironment to which an organism adapts itself.