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Show 118 NATURAL SELECTION. [OliAP. IV. Thus the sinall differences distinguishing v~rieties of the same species, will steadily tend to increase till ~hey come to equal the greater differences between speCies of the saine genus, or even of distinct genera. . We have seen that it is the cominon, the widely-diffused, and ·widely-ranging species, belonging to the larger genera, which vary most; and these will tend to transmit to their modified offspring that superiority which now rnakes them dominant in their own countries. N atural selection, as has just been ren1arked, leads to divergence of character and to much extinction of the less improved and intermediate forms of life. On these principles, I believe, the nature of the affinities of all organic beings may be explained. It is a truly wonderful fact-the wonder of which we are apt to overlook from familiarity -that all animals and all plants throughout all time and space should be related to each other in group subordinate to group, in the manner which we everywhere beholdnamely, varieties of the same species most closely related together, species of the same genus less closely and unequally related together, forming sections and sub-genera, species of distinct genera much less closely related, and genera related in different degrees, forming sub-families, families, orders, sub-classes, and classes. The several subordinate groups in any class cannot be ranked in a single file, but seem rather to be clustered round points, and these round other points, and so on in almost endless cycles. On the view that each species has been independently created, I can see no explanation of this great fact in the classification of all organic beings ; but, to the best of my judgment, it is explained through inheritance and the complex action of natural selection, entailing extinction and divergence of character, as we have seen illustrated in the diagram. The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species ; and those produced during each former year 1nay represent the long succession of extinct species. At each period of growth CHAP. IV.] SUMMARY. 119 all the growing twi~s have tried t b and· to overtop and I'll the surrouon d;Irnagn ct h ·o ut ond a ll sides ' es, In the same manner as s . Wigs an branch-have tried to overmaster oth!eCies .an~ groups of species forJife. The limbs divided . ~ speCies bn the great battle into lesser and lesser bran~~ o great rhnches, and these when the tree was small bu;J~ we;e. t emselves once, nection of the former a~d Ing Wigs ; and this conbranches may well represent tEre~ent.fibud_s by ranlifying and living species in groups e ~a~: cation of all extinct the many twigs which flourish~dor hnate to groups. Of mere bush, only two or th w en the tree was a b~anches, yet survive and bea~e~ll~hw fh'owbn into great Wit~ the species which lived durin e o er ranches ;_ so periods, very few now have r . g ~ong-past geological ants. From the first growth ~f~~ an. rnodified ~escendbranch has decayed and d ~tied many a hmb and branches of various siz roppe o ; and these lost orders, families and en es mal· represent those whole rep~esentatives,' and Jhicha a:: kh have now no living having been found in a fossil state noAn to hus only from see a thin straggling branch : . s ~e ere and there down in a tree and h. h spl'lnging Jrom a fork low favoured and is stlll ali~ ole 't by son;e chance has been see an anirnal like the 0 n :th shmmit, so we occasionally which in some small d rnl or ynchus .or Lepidosiren large branches of life e~~~e ch-nhcts by Its affinities tw~ saved from fatal competition b lh ~as .appa~ently been tected station .As bud . .Y bving Inhabited a pro-and these, if ;igorous, b~!~:h ~~~ Y dgrowth to fresh buds, many a feebler branch b an . overtop on all sides been with the great Tr~~oof i ~enerh~I<hn I believe it has and broken branches the cru~:' ~ h fills with its dead the. surface with its ever branch. o t ed ebarth, ~nd covers cations. Ing an eautiful ramifi- |
