GEOGRAPHICAL DISTRIBUTION 



141 



South America, the cattle having been fed on the 

 vovage with the pods belonging to the tree. 



thiT, the parasitic habits of certain ;tnim..l- 



emiMe I IH-III to I"- carried alwHit from place to 



place, when they have themselves no power, or 



onl\ a very feeble power of locomotion. And, with 



.1 to tin- subject now under consideration, it 



s no difference whether the animals are truly 



itir, feeding at the expense of the host to 



which they are attached, or merely commen&alwte, 



gamin;,' tlieir own food independently, like the sea- 



Hin-iiioiie-i so firi|tiently attached to the shells of 



hermit crabR. 



Lastly, man is often unintentionally the means 

 ..; conveying both plants and aninnils from one 

 M to another. The foreign plants found grow- 

 ing mi ballast-heaps are instances of this, and so 

 are the plants which have sprung from seed 

 introduced with imported grain or other articles of 

 import. Since the discovery of America the whole 

 of the northern part of the continent is said to 

 have l>een more or less overrun by European weeds ; 

 and, according to Agassiz, the roadside weeds of the 

 New England states, to the number of 130, are all 

 Kurt mean. Wherever European sailors have gone, 

 tli- Kuropean rats, both black and brown, have 

 accompanied them; and the shrew, the death's- 

 head moth, the Sphinx convolvuli, &c., are also 

 known to have oeen introduced into various 

 countries in ships. 



In the preceding summary of the more important 

 means of diffusion for plants .and animals, spine of 

 the obstacles to diffusion have been incidentally 

 referred to ; but it will be convenient to make a 

 general survey of these also. 



For all land-plants and land-animals the most 

 obvious and effective barrier is a wide expanse of 

 ocean ; and where the expanse is very wide it is 

 seldom passable except with the aid of man. For 

 land mammals the ocean is an absolutely im- 

 pa ^sahle barrier, and hence native mammals are 

 always absent from oceanic islands (i.e. islands 

 that have never been connected with the main- 

 land ) ; and this barrier is almost equally effective 

 for serpents and amphibians, which also are nearly 

 always wanting where there are no native mam- 

 mals. Lizards are more frequently found indigen- 

 ous on oceanic islands, though their means of 

 transit from the mainland is unknown. Arms of 

 the sea and broad rivers are likewise generally 

 impassable for the creatures mentioned, though 

 some of them have greater powers of swimming 

 than is generally supposed. The jaguar, the bear, 

 and the bison are capable of swimming the widest 

 rivers ; pigs have been known to swim ashore 

 when carried out to sea to a distance of several 

 miles ; and even a boa constrictor, it is said, has 

 swum to the island of St Vincent from the South 

 American coast a distance of 200 miles. 



Mountains, and especially high mountains, are 

 also frequently effective barriers to the migration 

 of land plants and animals ; but it must be noticed 

 i hat in some cases they serve for both as a means 

 ot communication between one region and another, 

 enabling plants and animals belonging to a cold 

 climate, for example, to spread into latitudes where, 

 in the plains, the climate is too hot for them. 

 Au'ain, deserts act as a barrier to the majority of 

 plant* and animals ; forests are a barrier to the 

 camel, hare, zebra, giraffe, &c. ; treeless regions to 

 apes, lemurs, and many monkeys ; plains to wild 

 goats and sheep. Broad rivers also act occasionally 

 as Iwirriers to distribution, and that, strange to 

 say, even in the case of some species of birds. 



Another important barrier is that of climate; 

 '"it, with reference to this, it must be observed that 

 the question of climate affects the problems of 

 geographical distribution, in the proper sense of 



that term, only in HO far at* climatic condition* 

 may shut off plants and animals from means of 

 communication between one region and another, 

 and not where climate merely limit- the range of 

 a species or group within a continuous area. In 

 the case of many animals climate acts only in- 

 directly as a barrier through limiting the food- 

 supply required bv them. 



Another set of barriers may be classed under the 

 general head of organic, inasmuch as they are all 

 connected with the vegetable or animal life of the 

 region where such barriers exist. Under this head 

 may be mentioned first the fact that certain 

 animals require for their subsistence a social 

 kind of vegetable food. The range of insects is 

 peculiarly liable to be limited in this way, certain 

 insects ix-ing attached to particular species of 

 plants, and others to genera or families ; and for 

 this reason insects, in spite of the exceptional 

 facilities for dispersal which, as we have already 

 seen, they enjoy, are remarkable, as a rule, rather 

 for the restriction of their areas of distribution 

 than for their wide diffusion. Again, the presence 

 of enemies is sufficient in some cases absolutely to 

 exclude certain forms from certain areas, as the 

 well-known tse-tse fly does horses, dogs, and cattle 

 from a well-defined area in South Africa ; and 

 another kind of fly prevents horses and cattle from 

 running wild in Paraguay, as they do in abundance 

 both to the north and south of that region. 



But a more important, because more generally 

 operative, organic uarrier consists in the fact of a 

 region being already fully occupied by a native 

 flora and fauna, so that there is no room for new- 

 comers. Hence it happens that seeds may be 

 wafted in plenty from one country to another 

 without a single plant growing from these seeds 

 being able to establish itself ; and there may even 

 be, as in South America, a free communication 

 with another region while the fauna remains 

 strikingly distinct, simply because that portion of 

 the American continent is already completely 

 stocked with a fauna perfectly adapted to the 

 physical conditions there prevailing. 



The barriers to the spread of marine creatures 

 are not so numerous as in the case of terrestrial 

 forms. The freedom of communication between 

 one part of the ocean and another makes it im- 

 possible to mark out any marine zoogeographical 

 regions, though many seas and coasts are dis- 

 tinguished by characteristic fishes and other marine 

 creatures. The principal barriers for fish are 

 temperature and the intervention of land. Thus, 

 the Isthmus of Panama is at present a complete 

 barrier for fishes requiring warm seas. 



If all the barriers to migration had existed in 

 all past time as they are now, it would be quite 

 impossible to explain the present distribution of 

 plants and animals on the supposition that kindred 

 groups have had a common birthplace. But the 

 solution of the problems of distribution is to be 

 found in the fact that all the barriers are liable 

 to change. Of changes of sea and land geology 

 supplies us with abundant evidence. Portions of 

 the mainland now continuous were at one time 

 severed by arms of the sea ; and islands have been 

 formed by the severance of portions of land that 

 once belonged to the mainland. Such islands are 

 known as continental islands, and the study of 

 their faunas and floras is one of peculiar interest 

 in connection with geographical distribution. 

 These faunas and floras show, as might be ex- 

 pected, a greater or less degree of correspondence 

 with those of the mainland from which the islands 

 have been cut off; and the resemblance is the 

 closer the more recently the land connection has 

 been destroyed. The relative date of the disunion 

 is usually approximately indicated by the depth 



