18 PEELIMINARY INQUIPvIES. 



it and of the neighbouring continent have been now for a considerable time minutely known ; and 

 although all the species in Britain are also found on the Continent, there arc many species in 

 the latter which are not found in Britain. The length of time for which authentic data have 

 existed is no doubt but a brief moment compared to the lapse of ages which Mr. Darwin requires for 

 his colonization ; but the thousand facilities for introduction which are daily furnished by the constant 

 intercom-se between England and the Continent may be allowed in some respect to compensate for 

 the briefness of our chronicle, and we may fairly argue from what has taken place durmg that time as 

 if it represented a much longer period. And what does it show ? Putting aside the introduction of 

 plants brought by commerce or in ballast, and ichicli almost invariably dimp2xar in the course of a 

 few years, how few colonists can Britain claim from the Continent during the past century. More 

 than that, the difficulty which a colony (much more a single colonist) must encounter in establishing 

 itself on a foreign shore may be estimated from the fact that any attempts which have been 

 made to introduce and naturalize species from the other side of the Channel, have always failed. 

 The difficulty which we exi^erienco whenever we attempt to transport animals from more distant 

 lands into other coimtrios, even if we secure for them as nearly as can be the same conditions as 

 those in which they used to live, is another instance of the same kind. The same thing may 

 be observed everywhere. Sir Charles Lyell says of America, " Many European plants arc making 

 their way here, such as the wild camomile and the thorn apple [Datura Stramonium) ; and it is 

 a curious fact, which I afterwards learned from Dr. Dale Owen, that when such foreigners are first 

 naturalised they overrun the country with amazing rapidity and are quite a nuisance. But they 

 soon grow scarce, and after eight or' ten years can hardly be met with."* The overrunniag phase 

 seems to be the stage to which New Zealand has arrived at the present time ; an immense irruption 

 of EurojDcan weeds has been made in these islands, and it will doubtless disappear there in a few 

 years, as has been the case in America. 



But much less important obstacles than straits or passages of the sea act as efifectual barriers 

 against distribution. Rivers also sometimes form effective barriers, although I believe that in 

 most instances where they so act the phenomenon has been originally occasioned by a former 

 different arrangement of land and sea, and the obstacle then occasioned b}' a strait or sea been 

 kept up by the river now flowing in their coui-se. This I beKeve to be the case with the Amazon 

 and Orinoko, which furnish many instances in point ; for example, Mr. Wallace mentions that 

 " on the north side of the Amazon, and east of the Rio Negro, are found the following 

 three species of monkeys : — Ateles paniscus, Brachyukus Satanas, and Jacchus bicoloh. These 

 are all found close uj) to the margins of the Rio Negro and Amazon, but never on the opposite 

 banks of either river." 



Again, a species of Pithecia is found on the west side of the Rio Negro for several hundred 

 miles from its mouth up to the River Curicuriari, but never on the east side ; neither is it known on 

 the south side of the Upper Amazon, where it is replaced by an allied species, the P. iiirok.\ta 

 (P. HIRSUTA Spix.), which, though abundant there, is never fomid on the north bank. He 

 mentions, also, that on the south side of the Lower Amazon, in the neighbourhood of Para, are 

 found two monkeys, Mycetes Beelzeisub and Jacciius tamarin, which do not pass the river to the 

 north. 



Mr. Bates bears similar testimony. He informs me that it is the universal and spontaneous 



* Ltell's " Second Visit to the United States," ii. 270, 1850. 



