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！
ributed to climate； food； &c。： thus； E。 Forbes speaks confidently that shells at their southern limit； and when living in shallow water； are more brightly coloured than those of the same species further north or from greater depths。 Gould believes that birds of the same species are more brightly coloured under a clear atmosphere； than when living on islands or near the coast。 So with insects； Wollaston is convinced that residence near the sea affects their colours。 Moquin…Tandon gives a list of plants which when growing near the sea…shore have their leaves in some degree fleshy； though not elsewhere fleshy。 Several other such cases could be given。
The fact of varieties of one species； when they range into the zone of habitation of other species； often acquiring in a very slight degree some of the characters of such species； accords with our view that species of all kinds are only well…marked and permanent varieties。 Thus the species of shells which are confined to tropical and shallow seas are generally brighter…coloured than those confined to cold and deeper seas。 The birds which are confined to continents are； according to Mr Gould； brighter…coloured than those of islands。 The insect…species confined to sea…coasts； as every collector knows； are often brassy or lurid。 Plants which live exclusively on the sea…side are very apt to have fleshy leaves。 He who believes in the creation of each species； will have to say that this shell； for instance； was created with bright colours for a warm sea； but that this other shell became bright…coloured by variation when it ranged into warmer or shallower waters。
When a variation is of the slightest use to a being； we cannot tell how much of it to attribute to the accumulative action of natural selection； and how much to the conditions of life。 Thus； it is well known to furriers that animals of the same species have thicker and better fur the more severe the climate is under which they have lived； but who can tell how much of this difference may be due to the warmest…clad individuals having been favoured and preserved during many generations； and how much to the direct action of the severe climate？ for it would appear that climate has some direct action on the hair of our domestic quadrupeds。
Instances could be given of the same variety being produced under conditions of life as different as can well be conceived； and； on the other hand； of different varieties being produced from the same species under the same conditions。 Such facts show how indirectly the conditions of life must act。 Again； innumerable instances are known to every naturalist of species keeping true； or not varying at all； although living under the most opposite climates。 Such considerations as these incline me to lay very little weight on the direct action of the conditions of life。 Indirectly； as already remarked； they seem to play an important part in affecting the reproductive system； and in thus inducing variability； and natural selection will then accumulate all profitable variations； however slight； until they become plainly developed and appreciable by us。
Effects of Use and Disuse
From the facts alluded to in the first chapter； I think there can be little doubt that use in our domestic animals strengthens and enlarges certain parts； and disuse diminishes them； and that such modifications are inherited。 Under free nature； we can have no standard of comparison； by which to judge of the effects of long…continued use or disuse； for we know not the parent…forms； but many animals have structures which can be explained by the effects of disuse。 As Professor Owen has remarked； there is no greater anomaly in nature than a bird that cannot fly； yet there are several in this state。 The logger…headed duck of South America can only flap along the surface of the water； and has its wings in nearly the same condition as the domestic Aylesbury duck。 As the larger ground…feeding birds seldom take flight except to escape danger； I believe that the nearly wingless condition of several birds； which now inhabit or have lately inhabited several oceanic islands； tenanted by no beast of prey； has been caused by disuse。 The ostrich indeed inhabits continents and is exposed to danger from which it cannot escape by flight； but by kicking it can defend itself from enemies； as well as any of the smaller quadrupeds。 We may imagine that the early progenitor of the ostrich had habits like those of a bustard； and that as natural selection increased in successive generations the size and weight of its body； its legs were used more； and its wings less； until they became incapable of flight。
Kirby has remarked （and I have observed the same fact） that the anterior tarsi； or feet； of many male dung…feeding beetles are very often broken off； he examined seventeen specimens in his own collection； and not one had even a relic left。 In the Onites apelles the tarsi are so habitually lost； that the insect has been described as not having them。 In some other genera they are present； but in a rudimentary condition。 In the Ateuchus or sacred beetle of the Egyptians； they are totally deficient。 There is not sufficient evidence to induce us to believe that mutilations are ever inherited； and I should prefer explaining the entire absence of the anterior tarsi in Ateuchus； and their rudimentary condition in some other genera； by the long…continued effects of disuse in their progenitors； for as the tarsi are almost always lost in many dung…feeding beetles； they must be lost early in life； and therefore cannot be much used by these insects。
In some cases we might easily put down to disuse modifications of structure which are wholly； or mainly； due to natural selection。 Mr。 Wollaston has discovered the remarkable fact that 200 beetles； out of the 550 species inhabiting Madeira； are so far deficient in wings that they cannot fly； and that of the twenty…nine endemic genera； no less than twenty…three genera have all their species in this condition！ Several facts； namely； that beetles in many parts of the world are very frequently blown to sea and perish； that the beetles in Madeira； as observed by Mr Wollaston； lie much concealed； until the wind lulls and the sun shines； that the proportion of wingless beetles is larger on the exposed Dezertas than in Madeira itself； and especially the extraordinary fact； so strongly insisted on by Mr。 Wollaston； of the almost entire absence of certain large groups of beetles； elsewhere excessively numerous； and which groups have habits of life almost necessitating frequent flight； these several considerations have made me believe that the wingless condition of so many Madeira beetles is mainly due to the action of natural selection； but combined probably with disuse。 For during thousands of successive generations each individual beetle which flew least； either from its wings having been ever so little less perfectly developed or from indolent habit； will have had the best chance of surviving from not being blown out to sea； and； on the other hand； those beetles which most readily took to flight will oftenest have been blown to sea and thus have been destroyed。
The insects in Madeira which are not ground…feeders； and which； as