Page:Catholic Encyclopedia, volume 5.djvu/739

 EVOLUTION

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EVOLUTION

In order first of all to obtain a just estimate of the influence of selection, it must be pointed out that not everything that is attributed to selection has origi- nated through selection. The origin of many pure breeds (e. g., of pigeons) is unknown, and cannot therefore without further investigation be ascribed to selection. Furthermore, many cultivated forms have arisen through crosses and segregation of characters, but not through merely strengthening individual characters. If we restrict our examination only to well attested facts, we find, first, that nothing new is brought about by selection; secondly that the maxi- mum amount in quantitative modification is obtained in a few generations (mostly in three to five) and that this amount can only be maintained through constant selection. In ease selection is stopped, a regression will follow proportional to the length of time required for the progress. In short, as far as facts teach us, new species do not arise by selection. But if qualita- tive changes were produced by some other cause, se- lection would probably be a potent principle in order to explain %vhy some peculiarities survive and others disappear. The question is: Whether changes in the environment may furnish such a cause. There can be no doubt that the environment does influence organ- isms and mould them in many ways. As proof of this we need only draw attention to the different forms of Alpine and valley plants, to the formation of the leaves of plants according to the himiidity, shadiness,

or sunniness of the habitat, to the influence of light and temperature on the formation of pigment and colouring of the surface, to the strange and consider- able differences produced, for instance, in knot- weeds by merely changing the environment, and so forth. But as far as actual experiments show, the changes of characteristics and niceties of adaptation go to and fro, as it were, without transgressing definite ranges of variation. Moreover, it is not at all clear how discontinuity of species could have arisen " by a continuoiLS environment, whether acting directly, as Lamarck would have it, or as a selective agent, as Darwin would have it" (Bateson), unless one takes into account the accidental destruction and isolation of intermediate forms.

In spite of these conclusions it has been assimied that individual differences might lead to the formation of new species under the continuous influence of nat- ural selection. Wasmann's well-known Dinarda- forms may serve as an example. The four forms of the rove-beetle, Dinanla, namely D. Mdrkeli, D. den- tntti, D. Hagensi and D. pygmaa, bear a certain rela- tion with regard to size to the four forms of ants, For- mica rufii, songuinea, exsecUi, fusco-rufibnrhix, and to their nests, in which they live as tolerated guests. D. Mdrkeli, which is 5 mm. long, dwells with F. ruja, which is comparatively large and builds spacious hill- nests. D. dcnUita, which is 4 mm. long, lives with F. ganguinen, which is comparatively large, but builds small earth-nests. D. Hngensi, which is 3-4 mm. long, lives with /•'. exsecta, which is smaller than F.

sanguinea, but builds a fairly roomy hill-nest. D. pygmwa, which is 3 mm. long, lives with F. fusco-rufi- barbis, which is relatively small and builds small earth- nests. Moreover, the three first-named ants are two- coloured (red and black), and so are the corresponding LHnarda. The last- named ant, how- ever, is of a more uniform dark col- our, as is also the corresponding Dinarda. Now comparative zoo- geography c o n - tains some indica- tions according to which the similar- ity of colour and proportion of size must be at tributed to actual adapta- tion. For (1) there are regions in Cen- tral Europe in which only F. san- guinea with D. den- lata, and F. rufa with D. Mdrkeli are found, whereas F. exseeta and F. rufibarbis do not harbour any Din- arda-forms at all. Secondly, there are districts in which the four forms of Dinarda are living with their four hosts and yet hardly ever showing transitional forms. Thirdly, in other parts there are more or less continuous intermediate forms, D. dentuta-Hagensi living with F. exsecta, and D. Hagensi-pygmcea living with F. fusco-rufibarbis. The nearer a Dinarda ap- proaches the form of D. pygmaa, the more frequently it is found with F. fusco-rufibarbis. To all this must be added, that the adaptation in general appears to have kept pace with the historical freeing of Central Europe from ice, though numerous exceptions must be explained by local circumstances, especially by iso- lation. Considering these facts, we are inclined to be- lieve that D. pygmaa especially presents an example of real adaptation in fieri, though this adaptation cannot be called a progressive one, since the more recent forms, Hagensi and pyg- mcea, are only smaller in size and of a more uniform colour. But at the same time it seems to us that the adaptation of the Dinarda cannot be considered as an example to illiLs- trate specific evo- lution, because, as wehaveshown else- where, there are many instances in nature — we men- tion ordy the races and other sub-divi- sionsof the human species — that like- wise present differ- ent degrees of adaptation far more pronounced than that found in the Dinarda, but which are not, and cannot on that account be, quoted as examples of the formation of new specific characters.

(2) Single Variations are presumably of far greater importance for the solution of the evolution problem

(Enotiiera C!ig