物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 12. GEOGRAPHICAL DISTRIBUTION--continued. Page 3
In considering the wide distribution of certain genera, we should bearin mind that some are extremely ancient, and must have branched offfrom a common parent at a remote epoch; so that in such cases therewill have been ample time for great climatal and geographical changesand for accidents of transport; and consequently for the migration ofsome of the species into all quarters of the world, where they mayhave become slightly modified in relation to their new conditions.There is, also, some reason to believe from geological evidence thatorganisms low in the scale within each great class, generally changeat a slower rate than the higher forms; and consequently the lowerforms will have had a better chance of ranging widely and of stillretaining the same specific character. This fact, together with theseeds and eggs of many low forms being very minute and better fittedfor distant transportation, probably accounts for a law which has longbeen observed, and which has lately been admirably discussed by Alph.de Candolle in regard to plants, namely, that the lower any group oforganisms is, the more widely it is apt to range.
The relations just discussed,--namely, low and slowly-changingorganisms ranging more widely than the high,--some of the species ofwidely-ranging genera themselves ranging widely,--such facts, asalpine, lacustrine, and marsh productions being related (with theexceptions before specified) to those on the surrounding low lands anddry lands, though these stations are so different--the very closerelation of the distinct species which inhabit the islets of the samearchipelago,--and especially the striking relation of the inhabitantsof each whole archipelago or island to those of the nearestmainland,--are, I think, utterly inexplicable on the ordinary view ofthe independent creation of each species, but are explicable on theview of colonisation from the nearest and readiest source, togetherwith the subsequent modification and better adaptation of thecolonists to their new homes.
SUMMARY OF LAST AND PRESENT CHAPTERS.
In these chapters I have endeavoured to show, that if we make dueallowance for our ignorance of the full effects of all the changes ofclimate and of the level of the land, which have certainly occurredwithin the recent period, and of other similar changes which may haveoccurred within the same period; if we remember how profoundlyignorant we are with respect to the many and curious means ofoccasional transport,--a subject which has hardly ever been properlyexperimentised on; if we bear in mind how often a species may haveranged continuously over a wide area, and then have become extinct inthe intermediate tracts, I think the difficulties in believing thatall the individuals of the same species, wherever located, havedescended from the same parents, are not insuperable. And we are ledto this conclusion, which has been arrived at by many naturalistsunder the designation of single centres of creation, by some generalconsiderations, more especially from the importance of barriers andfrom the analogical distribution of sub-genera, genera, and families.
With respect to the distinct species of the same genus, which on mytheory must have spread from one parent-source; if we make the sameallowances as before for our ignorance, and remember that some formsof life change most slowly, enormous periods of time being thusgranted for their migration, I do not think that the difficulties areinsuperable; though they often are in this case, and in that of theindividuals of the same species, extremely grave.
As exemplifying the effects of climatal changes on distribution, Ihave attempted to show how important has been the influence of themodern Glacial period, which I am fully convinced simultaneouslyaffected the whole world, or at least great meridional belts. Asshowing how diversified are the means of occasional transport, I havediscussed at some little length the means of dispersal of fresh-waterproductions.
If the difficulties be not insuperable in admitting that in the longcourse of time the individuals of the same species, and likewise ofallied species, have proceeded from some one source; then I think allthe grand leading facts of geographical distribution are explicable onthe theory of migration (generally of the more dominant forms oflife), together with subsequent modification and the multiplication ofnew forms. We can thus understand the high importance of barriers,whether of land or water, which separate our several zoological andbotanical provinces. We can thus understand the localisation ofsub-genera, genera, and families; and how it is that under differentlatitudes, for instance in South America, the inhabitants of theplains and mountains, of the forests, marshes, and deserts, are in somysterious a manner linked together by affinity, and are likewiselinked to the extinct beings which formerly inhabited the samecontinent. Bearing in mind that the mutual relations of organism toorganism are of the highest importance, we can see why two areashaving nearly the same physical conditions should often be inhabitedby very different forms of life; for according to the length of timewhich has elapsed since new inhabitants entered one region; accordingto the nature of the communication which allowed certain forms and notothers to enter, either in greater or lesser numbers; according ornot, as those which entered happened to come in more or less directcompetition with each other and with the aborigines; and according asthe immigrants were capable of varying more or less rapidly, therewould ensue in different regions, independently of their physicalconditions, infinitely diversified conditions of life,--there would bean almost endless amount of organic action and reaction,--and weshould find, as we do find, some groups of beings greatly, and someonly slightly modified,--some developed in great force, some existingin scanty numbers--in the different great geographical provinces ofthe world.
On these same principles, we can understand, as I have endeavoured toshow, why oceanic islands should have few inhabitants, but of these agreat number should be endemic or peculiar; and why, in relation tothe means of migration, one group of beings, even within the sameclass, should have all its species endemic, and another group shouldhave all its species common to other quarters of the world. We can seewhy whole groups of organisms, as batrachians and terrestrial mammals,should be absent from oceanic islands, whilst the most isolatedislands possess their own peculiar species of aerial mammals or bats.We can see why there should be some relation between the presence ofmammals, in a more or less modified condition, and the depth of thesea between an island and the mainland. We can clearly see why all theinhabitants of an archipelago, though specifically distinct on theseveral islets, should be closely related to each other, and likewisebe related, but less closely, to those of the nearest continent orother source whence immigrants were probably derived. We can see whyin two areas, however distant from each other, there should be acorrelation, in the presence of identical species, of varieties, ofdoubtful species, and of distinct but representative species.
As the late Edward Forbes often insisted, there is a strikingparallelism in the laws of life throughout time and space: the lawsgoverning the succession of forms in past times being nearly the samewith those governing at the present time the differences in differentareas. We see this in many facts. The endurance of each species andgroup of species is continuous in time; for the exceptions to the ruleare so few, that they may fairly be attributed to our not having asyet discovered in an intermediate deposit the forms which are thereinabsent, but which occur above and below: so in space, it certainly isthe general rule that the area inhabited by a single species, or by agroup of species, is continuous; and the exceptions, which are notrare, may, as I have attempted to show, be accounted for by migrationat some former period under different conditions or by occasionalmeans of transport, and by the species having become extinct in theintermediate tracts. Both in time and space, species and groups ofspecies have their points of maximum development. Groups of species,belonging either to a certain period of time, or to a certain area,are often characterised by trifling characters in common, as ofsculpture or colour. In looking to the long succession of ages, as innow looking to distant provinces throughout the world, we find thatsome organisms differ little, whilst others belonging to a differentclass, or to a different order, or even only to a different family ofthe same order, differ greatly. In both time and space the lowermembers of each class generally change less than the higher; but thereare in both cases marked exceptions to the rule. On my theory theseseveral relations throughout time and space are intelligible; forwhether we look to the forms of life which have changed duringsuccessive ages within the same quarter of the world, or to thosewhich have changed after having migrated into distant quarters, inboth cases the forms within each class have been connected by the samebond of ordinary generation; and the more nearly any two forms arerelated in blood, the nearer they will generally stand to each otherin time and space; in both cases the laws of variation have been thesame, and modifications have been accumulated by the same power ofnatural selection.