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异型维管束排列成同心环状，2～4轮。此药材是
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旅游团离境后，留下的游客若需要旅行社继续为其提供导游等服务，则需______。
5 Surprisingly enough, modern historians have rarely interested themselves in the history of the American South in the period before the South began to become self-con sciously and distinctively "Southern" —the decades after 1815. Consequently, the cuhural history of Britain’s North American empire in the seventeenth and eighteech centuries has been written almost as if the Southern colonies had never existed. The American culture that emerged during the Colonial and Revolutionary eras has been depicted as having been simply an extension of New England Puritan culture. However, Professor Davis has recently argued that the South stood apart from the rest of American society during this early period, following its own unique pattern of cultural development. The case for South ern distinctiveness rests upon two related premises: first, that the cultural similarities among the five Southern colonies were far more impressive than the differences, and sec ond, that what made those colonies alike also made them different from the other colo nies. The first, for which Davis offers an enormous amount of evidence, can be accepted without major reservations; the second is far more problematic. What makes the second premise problematic is the use of the Puritan colonies as a basis for comparison. Quite properly, Davis decries the excessive influence ascribed by his torians to the Puritans in the formation of American culture. Yet Davis inadvertently adds weight to such ascription by using the Puritans as the standard against which to assess the achievements and contributions of Southern colonials. Throughout, Davis focuses on the important, and undeniable, differences between the Southern and Puritan colonies in mo tives for and patterns of early settlement, in attitudes toward nature and Native Ameri cans, and in the degree of receptivity to metropolitan cultural influences. However, recent scholarship has strongly suggested that those aspects of early New England culture that seem to have been most distinctly Puritan, such as the strong reli gious orientation and the communal impulse, were not even typical of New England as a whole, but were largely confined to the two colonies of Massachusetts and Connecticut. Thus, what in contrast to the Puritan colonies appears to Davis to be pecul iarly Southern—acquisitiveness, a strong interest in politics and the law, and a tendency to cultivate metropolitan cultural models--was not only more typically English than the cultural patterns exhibited by Puritan Massachusetts and Connecticut, but also almost cer tainly characteristic of most other early modern British colonies from Barbados north to Rhode Island and New Hampshire. Within the larger framework of American colonial life, then, not the Southern--but the Puritan colonies appear to have been distinctive, and even they seem to have been rapidly assimilating to the dominant cultural patterns by the late Colonial period. The passage suggests that by the late Colonial period the tendency to cultivate met ropolitan cultural models was a cultural pattern that was______.
韧皮部外侧，木栓层内侧具异型维管束。此药材是
4 The evolution of sex ratios has produced, in most plants and animals with separate sexes, approximately equal numbers of males and females. Why should this be so Two main kinds of answers have been offered. One is couched in terms of advantage to popula tion. It is argued that the sex ratio will evolve so as to maximize the number of meetings be tween individuals of the opposite sex. This is essentially a "group selection" argu- ment. The other, and in my view correct, type of answer was first put forward by Fisher in 1930. This "genetic" argument starts from the assumption that genes can influence the relative numbers of male and female offspring produced by an individual carrying the genes. That sex ratio will be favored which maximizes the number of descendants an indi vidual will have and hence the number of gene copies transmitted. Suppose that the popula tion consisted mostly of females, then an individual who produced sons only would have more grandchildren. In contrast, if the population consisted mostly of males, it would pay to have daughters. If, however, the population consisted of equal numbers of males and females, sons and daughters would be equally valuable. Thus a one-to-one sex ratio is the only stable ratio; it is an "evolutionarily stable strategy. " Although Fisher wrote before the mathematical theory of games had been developed, his theory incorporates the essen tial feature of a game that the best strategy to adopt depends on what others are doing. Since Fisher’s time, it has been realized that genes can sometimes influence the chro mosome or gamete in which they find themselves so that the gamete will be more likely to participate in fertilization. If such a gene occurs on a sex-determining (X or Y) chromo some, then highly aberrant sex ratios can occur. But more immediately relevant to game theory are the sex ratios in certain parasitic wasp species that have a large excess of fe males. In these species, fertilized eggs develop into females and unfertilized eggs into males. A female stores sperm and can determine the sex of each egg she lays by fertilizing it or leaving it unfertilized. By Fisher’s argument, it should still pay a female to produce equal numbers of sons and daughters. Hamilton, noting that the eggs develop within their host--the larva of another insect--and that the newly emerged adult wasps mate immedi ately and disperse, offered a remarkably cogent analysis. Since only one female usually eggs in a given larva, it would pay her to produce one male only, because this one could fertilize all his sisters on emergence. Like Fisher, Hamilton looked for an evolutionarily stable strategy, but he went a step further in recognizing that he was looking for a strate gy. The author suggests that the work of Fisher and Hamilton was similar in that both scientists______.
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