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Show 42 DARWINISM CHAP. have been carefully described, so that we poss~s~ . a con~id~rable mass of information on the subject. Ut1hsmg this mformation we will now endeavour to crive some idea of the nature and extent of variation in the bspecies of animals and plants. "d d 1 It is very commonly objected that the WI esprea._ _an( constant variability which is admitted to be ~ charactcnst1c of domesticated animals aml cultivated plants 1s largely dnc to the unnatural conditions of their existence, a~d _that we h~vc no proof of any corresponding amount of varmt~o~ occ~nTmg in a state of nature. Wild animals and plants, It IS smd, arc usually stable, and when variations occur. these arc alleged to be small in amount and to affect superfic1al characters only; or if larger and more important, to oc_cnr so rarely a~ not to afford any aid in the supposed format10~1 of new spcc1es. This objection, as will be shown, 1s utterly unfonndcd_; but as it is one which crocs to the very root of the problem, 1t is necessary to enter at some length !nt? the various proof. of variation in a state of nature. Th1s 1s the more ncccs. ary because the materials collected by Mr. Darwin bearing on this question have never bc~n publis~e?, a~d co_mparati_vcly few of them have been cited m The Ongtn oj Specu s ; while a considerable body of facts has been made known since the publication of the last edition of that work. Variability of the Lower Animals. Amoncr the lowest and most ancient marine organisms arc the Fora~inifera, little rna scs of living jelly, apparenLly structnreless, but which secrete beautiful shelly coverin gs, often perfectly symmetrical, as varied in form :ts those of ~ h ~· mollusca. and far more complicated. These have hccn stnd1 ed with 1rrcat care by many eminent naturalists, and the late I )r. \V. B~ Carpenter in his great work-the Int?·odu ction to the Study of the Fomminifem-thus refers to their va.riahili ty: "There is not a sincrlc species of plant or animal of which the b 0 l mncrc of variation has been studied by the colloca.twn an< cor:parison of so large a number of specimens as have pass d under the review of Messrs. \Villiamson, Parker, Rupert Jones, and myself in our studies of the types of this cr:oup ; " and he states as the result of this extensive companson of III VARIABILITY OF SPECIES IN A STATE OF NATURE 43 specimens : "The range of variation is so great among the Foraminifera as to include not merely those differential characters which have been usually accounted specific, but also those upon which the greater part of the genera of this group have been founded, and even in some instances those of its orders." 1 . Coming now to a higher group-the Sea-Anemones-Mr. P. H. Gosse and other writers on these creatures often refer to variations in size, in the thickness and length of the tentacles, the form of the disc and of the mouth, and the character of surface of the column, while the colour varies enormously in a great number of the species. Similar variations occur in all the various groups of marine invertebrata, and in the great sub-kingdom of the mollusca they arc especially numerous. Thus, Dr. S. P. Woodward states that many present a most perplexing amount of variation, resulting (as he supposes) from supply of food, variety of depth and of saltness of the water; but w~ know that many variati0l1s are quite independent of such causes, and we will now consider a few cases among the land-mollusca in which they have been more carefully studied. In the small forest region of Oahu, one of the Sandwich Islands, there have been found about 17 5 species of land-shells represented by 700 or 800 varieties; and we arc told by the Rev. J. T. Gulick, who studied them carefully, that "we frequently find a genus represented in several successive valleys by allied species, sometimes feeding on the same, sometimes on different plants. In every such case the val1eys that arc nearest to each other furnish the most nearly a1lied forms ; (£1Ul a J~tll set of the vm·ielies of each species presents a minute gmdation of fO?·ms between the m01·e divm·gent types found in the more widely sepnrated localities." In most land-shells there is a considerable amount of variation in colour, markings, size, form, and texture or striation of the surface, even in specimens collected in the same locality. Thus, a French author has enumerated no less than 198 varieties of the common wood -snail (Helix nemoralis), while of the equally common garden-snail (Helix hortensis) ninetyvarietieshave beendcscribed. Fresh-water sheUs are also 1 Foraminifem, preface1 p. x. |