按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
a and thus have been destroyed。
The insects in Madeira which are not ground…feeders； and which； as the flower…feeding coleoptera and lepidoptera； must habitually use their wings to gain their subsistence； have； as Mr。 Wollaston suspects； their wings not at all reduced； but even enlarged。 This is quite compatible with the action of natural selection。 For when a new insect first arrived on the island； the tendency of natural selection to enlarge or to reduce the wings； would depend on whether a greater number of individuals were saved by successfully battling with the winds； or by giving up the attempt and rarely or never flying。 As with mariners ship…wrecked near a coast； it would have been better for the good swimmers if they had been able to swim still further； whereas it would have been better for the bad swimmers if they had not been able to swim at all and had stuck to the wreck。
The eyes of moles and of some burrowing rodents are rudimentary in size； and in some cases are quite covered up by skin and fur。 This state of the eyes is probably due to gradual reduction from disuse； but aided perhaps by natural selection。 In South America； a burrowing rodent； the tuco…tuco； or Ctenomys； is even more subterranean in its habits than the mole； and I was assured by a Spaniard； who had often caught them； that they were frequently blind； one which I kept alive was certainly in this condition； the cause； as appeared on dissection； having been inflammation of the nictitating membrane。 As frequent inflammation of the eyes must be injurious to any animal； and as eyes are certainly not indispensable to animals with subterranean habits； a reduction in their size with the adhesion of the eyelids and growth of fur over them； might in such case be an advantage； and if so； natural selection would constantly aid the effects of disuse。
It is well known that several animals； belonging to the most different classes； which inhabit the caves of Styria and of Kentucky； are blind。 In some of the crabs the foot…stalk for the eye remains； though the eye is gone； the stand for the telescope is there； though the telescope with its glasses has been lost。 As it is difficult to imagine that eyes； though useless； could be in any way injurious to animals living in darkness； I attribute their loss wholly to disuse。 In one of the blind animals； namely； the cave…rat； the eyes are of immense size； and Professor Silliman thought that it regained； after living some days in the light； some slight power of vision。 In the same manner as in Madeira the wings of some of the insects have been enlarged； and the wings of others have been reduced by natural selection aided by use and disuse； so in the case of the cave…rat natural selection seems to have struggled with the loss of light and to have increased the size of the eyes； whereas with all the other inhabitants of the caves； disuse by itself seems to have done its work。
It is difficult to imagine conditions of life more similar than deep limestone caverns under a nearly similar climate； so that on the common view of the blind animals having been separately created for the American and European caverns； close similarity in their organisation and affinities might have been expected； but； as Schi？dte and others have remarked； this is not the case； and the cave…insects of the two continents are not more closely allied than might have been anticipated from the general resemblance of the other inhabitants of North America and Europe。 On my view we must suppose that American animals； having ordinary powers of vision； slowly migrated by successive generations from the outer world into the deeper and deeper recesses of the Kentucky caves； as did European animals into the caves of Europe。 We have some evidence of this gradation of habit； for； as Schi？dte remarks； 'animals not far remote from ordinary forms； prepare the transition from light to darkness。 Next follow those that are constructed for twilight； and； last of all； those destined for total darkness。' By the time that an animal had reached； after numberless generations； the deepest recesses； disuse will on this view have more or less perfectly obliterated its eyes； and natural selection will often have effected other changes； such as an increase in the length of the antennae or palpi； as a compensation for blindness。 Notwithstanding such modifications； we might expect still to see in the cave…animals of America； affinities to the other inhabitants 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…animals； as I hear from Professor Dana； and some of the European cave…insects are very closely allied to those of the surrounding country。 It would be most difficult to give any rational explanation of the affinities of the blind cave…animals to the other inhabitants of the two continents on the ordinary view of their independent creation。 That several of the inhabitants of the caves of the Old and New Worlds should be closely related； we might expect from the well…known relationship of most of their other productions。 Far from feeling any surprise that some of the cave…animals should be very anomalous； as Agassiz has remarked in regard to the blind fish； the Amblyopsis； and as is the case with the blind Proteus with reference to the reptiles of Europe； I am only surprised that more wrecks of ancient life have not been preserved； owing to the less severe competition to which the inhabitants of these dark abodes will probably have been exposed。
Acclimatisation
Habit 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 this leads me to say a few words on acclimatisation。 As it is extremely common for species of the same genus to inhabit very hot and very cold countries； and as I believe that all the species of the same genus have descended from a single parent； if this view be correct； acclimatisation must be readily effected during long…continued descent。 It is notorious that each species is adapted to the climate of its own home： species from an arctic or even from a temperate region cannot endure a tropical climate； or conversely。 So again； many succulent plants cannot endure a damp climate。 But the degree of adaptation of species to the climates under which they live is often overrated。 We may infer this from our frequent inability to predict whether or not an imported plant will endure 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 state of nature are limited in their ranges by the competition of other organic beings quite as much as； or more than； by adaptation to particular climates。 But whether or not the adaptation be generally very close； we have evidence； in the 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； raised from seed collected by Dr Hooker from trees growing at different heights on the Himalaya were found in this country to possess different c