DARWINIAN THEORY 



661 



in termed Natural Si-l.-,-ti,.n, or lew* figuratively, 

 tin- Survival of the Fittest. 



Taking the ease of a country undergoing a 

 cli.iii-.- i climate, the proportional Bombm of 

 ii- denizens would change, some sjiecies probably 



1 Mining extinct and these changes would in 



many ways affect the survivors. A further dis- 

 turbance \\.nilil .-.HII,- from the immigration of new 

 forms ; or if that were prevented, we should have 

 places in the economy of nature which might be 

 better filled up. Any slight favourable modifica- 

 tion of the old species would tend to be preserved, 

 and we have seen that changed conditions increase 

 variability. 



Nor are such changes necessary in order to leave 

 places for natural selection to fill. No country can 

 be named where the native inhabitants are per- 

 fectly adapted to their conditions and competitors, 

 for as some foreigners have taken firm possession 

 in every country, we may safely conclude that the 

 natives might have been modified with advantage 

 to resist them. 



And when human selection has produced such 

 great results, why may not natural? Human 

 selection acts, only for man's own good, on mere 

 external and visible characters, and irregularly 

 throughout a short period ; natural selection acts 

 for the good of the being itself, on the whole 

 machinery of its life, and incessantly on the 

 species, throughout almost infinite time. (It is 

 important here to remember that the objection to 

 this agency on the ground of its presumed insigni- 

 ficance, is identical with that so long but unsuc- 

 cessfully employed against Lyell's explanation of 

 the origin of the physical features of the globe by 

 summing up the existing natural changes.) 



Natural selection thus leads to the improvement 

 of each creature in relation to its organic and 

 inorganic conditions of life, and consequently in 

 most cases to wliat must be regarded as an advance 

 in organisation. Nevertheless, low and simple 

 forms will long endure, if well fitted for their 

 simple conditions. 



Natural selection may modify the egg, seed, or 

 young, as easily as the adult, and these modifica- 

 tions may effect through correlation the structure 

 of the latter, and conversely. 



Besides Natural, we have to consider Sexual 

 Selection i.e. not merely do individuals struggle 

 for existence, but the males struggle for the 

 females, and the most vigorous thus tend to leave 

 most progeny. Special weapons, offensive and 

 defensive, like the cock's spurs, the stag's horns, 

 or the lions mane, are used in this struggle, 

 and the most useful variations are those which 

 are transmitted. Again, just as man can in a 

 short time give beauty to his domestic birds, so 

 there is no good reason to doubt that female birds 

 in thousands of generations, by selecting, as they 

 are observed to do, the most melodious or beauti- 

 ful males, might produce a marked effect, and 

 many sexual differences are thus explained. 



The theory of natural selection may be applied 

 in special cases e.g. ( 1 ) to explain the evolution 

 of swift greyhound-Tike varieties of wolves; (2) to 

 explain tue origin and the excretion of nectar in 

 flowers, its use to insects, the action of insects in 

 transferring pollen from flower to flower, with its 

 advantage in intercrossing ; and the resultant 

 modification and adaptation of flower and insect 

 to each other by the preservation of advantageous 

 variations. 



The circumstances favourable to the production 

 of new forms through natural selection are also 

 reviewed. These are chiefly, great variability ; 

 large numbers of individuals ; the complex effects 

 of intercrossing ; isolation in small areas, vot also 

 extension over continental ones, especially if these 



vary in altitude ; and considerable lapse of time. 

 Hare species are shown to be in procen* of extnx 

 tion. Tli- <lii-ergence of character in domestic 

 breeds, largely due to the fact that ' fancier* do 

 not, and will not, admire a medium ntandard, but 

 like extremes,' applies throughout nature, from the 

 circumstance that the more diversified the descend- 

 ants from any one species become in structure, 

 constitution, and habits, by so much will they 

 lie better enabled to seize on many and widely 

 diversified places in nature, and so to increase in 

 numbers. Thus, taking a carnivorous animal which 

 has reached the maximum numbers its territory 

 will support, it is evident that it can succeed in 

 increasing only by its varying descendants seizing 

 places hitherto occupied by other animals. This 

 must hold equally of all species, and is separately 

 demonstrated for plants. The greatest amount of 

 life can be supported by help of proportionally great 

 diversification of structure ; hence, in small areas 

 where competition is severe, the inhabitants are 

 extremely varied. 



The probable effects of the action of Natural 

 Selection, through divergence of character and 

 extinction, on the descendants of a common 

 ancestor are then discussed in detail by Mr Darwin 

 with an illustrative diagram. This takes the form 

 of a genealogical tree ' the great tree of life, which 

 fills with its dead and broken branches the crust of 

 the earth, and covers the surface with ite ever- 

 branching and beautiful ramifications.' 



Laws of Variation (see VARIATION ). Of the 

 cause of most variations we are still ignorant, but 

 the same laws appear to have acted in producing 

 the lesser differences between varieties of the same 

 species and the greater differences between species 

 of the same genus. Changed conditions sometimes 

 induce definite and permanent effects : habit, u.-r. 

 and disuse are potent in their effects. Specific 

 characters are more variable than generic, and 

 varietal than either. Kudimentary organs and 

 secondary sexual characters are highly variable. 

 Species closely related of similar constitution and 

 similarly influenced, present analogous variations, 

 and frequently exhibit characters which can only 

 be explained as reversions to those of their ancient 

 progenitors e.g. zebra-like stripes on horses, or 

 wood-pigeon's markings on fantails, tumblers, &c. 



Difficulties and Objection. In four chapters all 

 the miscellaneous objections raised against the 

 theory between 1859 and the appearance of the 

 latest edition are successively stated, weighed, dis- 

 cussed, and met, as well as the much more serious 

 difficulties pointed out by Darwin himself. These 

 latter are, (1) the definiteness of species and the 

 rarity of transitional forms ; ( 2 ) the enormous 

 degree of modification in habits and structure 

 assumed by the theory, and the seeming improba- 

 bility that Natural Selection should produce on the 

 one hand an organ of such trifling importance aa 

 the tail of a giraffe, and on the other, an organ MI 

 wonderful as the eye; (3) the acquirement and 

 modification through Natural Selection of such 

 marvellous instincts as thoxe of the bee ; ( 4 ) the 

 sterility of crossed species, and the fertility of 

 crossed varieties. For these discussions, however, 

 the reader must consult the work itelf. 



IiK/ifi'/crtmn of the Geological Hccord. On the 

 assumption of the extermination of an enormous 

 numlwr of intermediate varieties, which were the 

 links between existing and remote ancestral forms 

 why, then, is not every geological formation 

 charg'ed with such links ?* Why does not every 

 collection of fossils afford plain evidence of the 

 gradation and mutation of the forms of life? 

 Geology, assuredly, does not reveal any such finely 

 graduated organic chain, and this is one of the 

 most obvious and plausible objection* to the 



