OCR Text |
Show 150 LAWS OF VARIATION. [CUAP. V. form or character. When the oldest and truest breeds of various colours are crossed, ·we see a strong t~ndency for the blue tint and bars and marks to reappear 1n the nlono- rels. I have stated that tho 1nost probable hypothesis to ~ccount for the reappearance of very ancient characters, is-that there is a tendency in the young of each successive O'enoraHon to produce tho long-lost character, and b • that this tendency, fr01n unknown causes, sometnnes pre-vails. And we have just seen that in several species of tho horse-genus the stripes arc either plainer or appear 1nore commonly in tho young than in the old. Call tho breeds of pigeons, son1e of which have bred true for centuries, species; and how exactly parallel is tho case ·with that of the species of the horse-genus ! For 1nyself, I venture confidently to look back thousands on thousands of generations, and I sec an ani1nal strjpod like a zebra, but perhaps otherwise very differently constructed, the co1nn1on parent of our donwstic horse, ·whether or not it be descended fro1n one or more wild stocks, of the ass, the hemionus, quagga, and zebra. He who believes that each equine species ·was independently created, will, I presun1e, assert that each species has been created with a tendency to vary, both nuder nature and under domestication, in this particular n1anner, so as often to become striped like other species of the genus ; and that each has been created with a strong tendency, when crossed with species inhabiting distant quarters of the world, to produce hybrids resembling in their stripes, not their own parents, but other species of the genus. To admit this view is, as it seems to 1ne, to reject a real for an unreal, or at least for an unknown, cause. It 1nakes the works of God a mere mockery and deception; I would almost as soon beHevc with the old and ignorant cosmogonists, that fossil shells had never lived, but had been created in stone so as to mock the shells now living on the sea-shore. Summary.-Our ignorance of the laws of variation is profound. Not in on~ case out of a hundred can we Ou.A.P. V.] SUMMARY. 151 P!e~tend to assign any reason why this or that part d1ifers, 1nore or less, from tho sa1ne part in tho parents. Tiu~ whenever w·o have the 1neans of instituting a COlllpanson, the sa1ne laws appear to have acted in producin othe lesser differences bet\veen varieties of the same sp~ cios, and the greater differences between species of tho same genus. Tho external conditions of life, as clhnate and food, &c., semn to have induced some slight Inodifications. IIabit in producing constitutional differences and "?-s~ in strengthening, and disuse in weakening and diminlslnng organs, semn to have been rnore potent in their effects. Iloinologous parts tend to vary in the sa1ne way, and homologous parts tend to cohere. Modifications in hard parts and in external parts smnetiines affect softer and internal parts. When one part is largely developed, perhaps it tends to draw nourislunent from the adjoining parts; and every part of the structure which can be saved without detriment to the individual, will be saved. Changes of structure at an early age will generally affect parts subsequently developed ; and there are very many other correlations of growth, the nature of which we are utterly unable to understand. Multiple parts arc variable in number and in structure, perhaps arising fro1n such parts not having been closell specialised to any particular function, so that their modifications have not been closely checked by natural selection. It is probably from this same cause that organic beings low in the scale of nature are n1ore variable than those which have their whole orga~isation 1nore specialised, and arc higher in tho scale. Rudimentary organs, frmn being useless, will be disregarded by natural selection, and hence probably are variable. Specific characters-that is, the characters which have como to differ since the several species of the saine genus branched off fro1n a co1nmon parent-are more variable than generic characters, or those which have long been inherited, and have not differed ·within this same period. In these remarks we have referred to special parts or organs being still varia~l~, 1)~cause they have recently vaned and thus come to differ; but we have also seen in the second Chapter that the same principle applies |