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Show 126 LAWS OF VARIATION. [OJIAP. V. It is difficult to n. nag1. ne cond1. ti·o ns of 11·£ e molr· e si. mi.lar . d a near y simi1 a r than deep hmeston caverns u? er . d animals Cll.rrlate · so that on the common vww of the bhn . d havinO' ' been separately crea t ed 1.{.' 0r the A. merJCa.n ta• n Europoe an caverns, close S·l ml·1 ari• t y I·l l .t hmr orgaSn 1hsa' · ·1d,0t n and affinities might have been expected; but, as c doth e and others have remarked, this is not the case, an e -insects of the two continents are not more closely cnT:d than might have been anticipated from the gene~·al :esemblance of the other jnhabitants of North Amenca and Europe. On my view we must suppose. ~hat Amer1. ean an1· ma ls , hav1'ng ordinary. powers ofh vision, .s lowlldy miO'rated by successive generations fron1 t e outm wor · t~ the deeper and deeper recesses of the ICentucky caves, ~~ did European animals i~to the ca:ves of Eu~o~e. We have some evidence of this gradation of hab1t, f?r, as Schiodte remarks, " animals. not far ren~ote froin ordinary fonns, prepare the transition from hght t?. darkness. Next follow those that are constructed for tw1hght ; and, last of all those destined for total darkness." By the time that ~n animal had reached, aft~r numb~rle~s generations, the deepest recess~s, disus~ Will on th1s view have more or less perfectly obhterated Its eyes, and natural selection will often have effected other changes2 such as an increase in the length of the an~ennre o~ pal pi, as a ?ompensation for blindness. N otwith~tand1ng such J?Odlfications, we might expect still to se~ In t~e cave-animals of America, affinities to the other Inhabi:ants .of that continent and in those of Europe, to the Inhabitants of the European continent. And this is the case with some of the American cave-anirnals, as I hear from Professor Dana· and some of the European cave-insects are very closely allied to those of the sui-roun~ing country .. It would be most difficult to give any rational explanatl?n of the affinities of the blind cave-animals to the other Inhabitants of the two continents on the ordinary v}ew of their independent creation. That several of the mhabltants of the caves of the Old and New Worlds should be closely related, we might. expect from the. well-known relationship of most of therr other productions. Far from CJIAP. V.] .A OOLIMAJ'ISAT}:ON. 121 feeling any surprise that some of the cave-animals should be ve~y anOinalous, as Agass~z has rema~ked in regard to the bhnd fish, the Amblyopsis, and as is the case with the blind Proteus .with reference to the re~tiles. of Europe, I am only surpnsed that more wrecks of ancient life have not b~en pre~erve~, owinO' to the less severe competition to whiCh the Inhabitants of these dark abodes will probably l1ave been exposed. . Accli'lr!'atisation.-~abi.t is hereditary with plants, as In the period of flowering, In the amount of rain requisite for seeds to germinate, in the time of sleep, &c., and thjs leads me to say a few words on acclin1atisation. As it is exti:ernely common for species of th~ same genus to inhabit very hot and very cold countries, and as I believe that all the species of the same genus have descended frorn a single parent, if this view be correct, acclimatisation must be readily effected during long-continued desce~t. It is ~otorious that each ~pecies is adapted to the clin1ate of Its own home : spemes from an arctic or e':en from a temperate region .cannot endure a tropical · climate, or conversely. So again, many succulent plants ca~not endur~ a damp cli~nate. But the degree of adaptation of spemes to the cln~ates u~der whicli they live is oft.e~ overrate~. We may Infer this from our frequent inability to predict whether or not an imported plant will cn~ure our climate, and from the number of plants and animals brought from warmer countries which here enjoy good health. We have reason to believe that species in a. s~~te of nature are li~ited. in their ranges by the competition of other organic bmngs quite as much as or more than, by adaptation to particular climates. But whether or. not th~ adaptation be generally very close, we have ~vidence, In tl:e case of some few plants, of their becomIng, to a certain extent, naturally habituated to different temperatures, or becoming acclimatised: thus the pines and rhododendrons, ~aised fr.om seed c?llected by Dr. Hooker from trees g~owi~g at different hmghts on the Himalaya, '!ere found In this country to possess different constitu· bonal powers of resisting cold. .Mr. Thwaites informs me |