[单项选择]Non-indigenous (non-native) species of plants and animals arrive by way of two general types of pathways. First, species having origins outside the United States may enter the country and become established either as free-living populations or under human cultivation-for example, in agriculture, horticulture, aquaculture, or as pets. Some cultivated species subsequently escape or are released and also become established as free-living populations. Second, species of either U.S. or foreign origin and already within the United States may spread to new locales. Pathways of both types include intentional as well as unintentional species transfers. Rates of species movement driven by human transformations of natural environments as well as by human mobility-through commerce, tourism, and travel-greatly exceed natural rates by comparison. While geographic distributions of species naturally expand or contract over historical time intervals (tens to hundreds of years), species’’ ranges rarely
A. Human factors affect its rates more than its long-term amount.
B. Natural expansions of species account for their slow contractions.
C. Natural environments created by humans facilitate species movement.
D. Long-range species movement relies on the ranges of man’’s mobility.

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[简答题] What is a Species A species of plants or animals, as the term has been generally used by naturalists, comprises all such individuals as are so similar to each other that we may suppose them all to have proceeded from one common parentage, and so dissimilar from all others that they could not have been reproduced from the others, nor the others produced from them. All we know is, that the plants and animals throughout the world exist in species, each one of which stands at present distinct and isolated wholly apart from all the rest, and one cannot be transformed into another by ordinary generation, through changes of soil and climate, or any other causes whatever known to man, within so short a period as six thousand years. The apple, for instance, is one species, and the pear is another. In many respects they are similar to each other, and each may be changed by cultivation and by the operation of other causes a great deal, but by no po

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