本篇文章共分 6 段，針對生物的分類學所面對到的基本問題進行分析與說明，並指出生物分類的困難之處與未來可能進行的不斷修正。

本篇考題英文原文與對應之中文翻譯整理如下。練習作答解題時若有對語意不清楚之處，請仔細查閱對照，以提升閱讀理解能力。

Taxonomy of Organisms 生物分類學

1. 分類學中的五界

   Classification schemes are used by biologists to place the huge number of organisms on Earth into natural groupings. Ideally, these groupings are made by taxonomists on the basis of shared distinguishing features. Today taxonomists use such features as anatomy, developmental stages, and biochemical similarities to categorize organisms. Early classification schemes placed all organisms into either the plant or animal kingdom. Later, close examination of the unique structure of fungi and the diversity of single-celled organisms made it necessary to propose additional kingdoms that recognized the fundamental differences among plants, animals, fungi, and unicellular prokaryotes (organisms whose cells do not have a distinct membrane-bound nucleus) and eukaryotes (organisms whose cells have a distinct membrane-bound nucleus). The current scheme consisting of five kingdoms—Monera, Protista, Animalia, Fungi, and Plantae—was devised in response to this need.

   生物學家使用分類方法將地球上數量龐大的生物體歸入自然組別。理想情況下，這些分組是由分類學家根據共同的區分特徵進行。今天，分類學家使用諸如解剖學、發育階段和生物化學的相似性等特徵來對生物進行分類。早期的分類方法將所有生物都歸入植物或動物界。後來，對真菌的獨特結構和單細胞生物的多樣性的仔細研究，使得有必要提出更多的界，承認植物、動物、真菌和單細胞原核生物（其細胞沒有明顯的膜結合的細胞核的生物）和真核生物（其細胞有明顯的膜結合的細胞核的生物）之間的根本區別。目前由五個界組成的體系—單胞類、原生類、動物類、真菌類和植物類—是為了滿足這一需要而設計的。

2. 物種的數量

   Scientists do not know how many species share our world. Each year 7,000 to10,000 new species are named. The total number of named species is currently around 1.4 million. However, many scientists believe that 7 million to 10 million species may exist, and estimates range as high as 30 million. Of all of the species that have been identified, about 5 percent are in the Monera and Protista kingdoms. An additional 22 percent are plants and fungi, and the rest are animals. This distribution has little to do with the actual abundance of these organisms and a lot to do with the size of the organisms, how easy they are to classify, and the number of scientists studying them.

   科學家們不知道有多少物種分享我們的世界。每年有 7,000 到 10,000 個新物種被命名。目前命名的物種總數約為 140 萬。然而，許多科學家認為，可能存在 700 萬到 1,000 萬個物種，甚至估計高達 3,000 萬。在所有已確定的物種中，約有 5% 是在單細胞和原生動物界。另外 22% 是植物和真菌，其餘是動物。這種分佈與這些生物的實際數量沒有什麼關係，反而與生物的大小、是否容易分類以及研究它們的科學家的數量有很大關係。

3. 分類標準制定的困難

   The kingdom Protista, defined as comprising all single-celled eukaryotic organisms, is not a natural grouping and scientists disagree about which organisms it should include. Plants, animals, and fungi all have close protistan relatives and the separation of single-celled organisms from multicellular organisms is sometimes problematic. It is especially so for the algae, which have both single-celled and multicellular representatives within most smaller taxonomic groupings. Can closely related organisms be placed into separate kingdoms, Protista and Plantae, simply on the basis of multicellula? If you look at different textbooks, you will see that the algae, photosynthetic organisms with simple reproduction, are sometimes placed entirely into Protista, and sometimes they are split between Protista and Plantae depending on whether they are single celled or multicellular. Some taxonomists split the multicellular algae into two kingdoms, placing the multicellular brown and red algae with the protists and the multicellular green algae into the plant kingdom. These different attempts to classify closely related organisms are good examples of how difficult it is to develop standard criteria for grouping organisms, even at the kingdom level.

   原生生物界被定義為包括所有的單細胞真核生物，這並不是一個自然的分組，科學家們對它應該包括哪些生物存在分歧。植物、動物和真菌都有密切的原生生物親屬，單細胞生物與多細胞生物的分離有時是有問題的。對於藻類來說更是如此，在大多數較小的分類組別中，藻類都有單細胞和多細胞的代表。僅僅根據多細胞，就可以將密切相關的生物體放入不同的界，即原生動物和植物界嗎？如果你看一下不同的教科書，你會看到藻類，具有簡單繁殖能力的光合生物，有時被完全歸入原生動物，有時根據它們是單細胞還是多細胞，被分割在原生動物和植物界之間。一些分類學家將多細胞藻類分成兩個界，將多細胞的褐藻和紅藻與原生動物放在一起，將多細胞的綠藻放入植物界。這些對密切相關的生物進行分類的不同嘗試是很好的例子，說明制定生物分類的標準是多麼困難，即使是在界級別。

4. 原生生物界

   One approach to this problem, enthusiastically endorsed by Lynn Margulis, a biologist at the University of Massachusetts, is the creation of the kingdom Protoctista. This taxonomic category would include single-celled organisms and their close descendants (for example, the multicellular algae but not the animals, fungi, and plants). Margulis describes the kingdom Protoctista as “the entire motley and unruly group of nonplant, nonanimal, nonfungal organisms representative of lineages of the earliest descendants of the eukaryotes.”

   馬薩諸塞大學的生物學家連恩馬古利斯熱烈支持解決這個問題的一種方法，即創建原生生物界。這個分類類別將包括單細胞生物和它們的近親（例如，多細胞藻類，但不包括動物、真菌和植物）。馬古利斯將原生生物界描述為「整個雜亂無章的非植物、非動物、非真菌生物群，代表了真核生物最早的後代的品系」。

5. 藻類的分類

   It is conceptually difficult to group one of the largest multicellular organisms in the world, the brown algae called giant kelps, with simple microscopic single-celled organisms. Kelps, some of which are up to 60 meters long, possess a tissuelike level of organization that is relatively complex and can transport materials over long distances, as can the tissues of higher plants. The cells in kelps and some other algae are specialized and show division of labor. However, kelps reproduce like other algae and differently from plants. Thomas Cavalier-Smith of the University of British Columbia has proposed that brown algae merit their own kingdom (kingdom Chromista) based on ultrastructural features and molecular comparisons of all algae. So, even among the algae, there are clear differences that some scientists believe are sufficient to justify the status of separate kingdom.

   從概念上講，將世界上最大的多細胞生物之一，即被稱為巨型海帶的褐藻，與簡單的微觀單細胞生物進行分組是很困難的。海帶，其中一些長達 60 米，擁有類似組織層級相當複雜的結構，可以長距離運輸物質，就像高等植物的組織一樣。海帶和其他一些藻類的細胞是特殊化的，顯示出分工。然而，海帶的繁殖方式與其他藻類一樣，但與植物不同。英屬哥倫比亞大學的托馬斯．卡瓦列爾．史密斯指出，根據所有藻類的超微結構特徵和分子比較，褐藻應該有自己的界（色藻界）。因此，即使在藻類中也存在有明顯的差異，因而一些科學家認為這足以證明獨立成界的地位。

6. 分類的不斷修正

   As we learn more about the relationships between organisms and refine the criteria used to classify them, classification schemes will change. As the superficially simple question "In which kingdom should we place the algae?" illustrates, the taxonomic categories in textbooks are tentative and subject to revision as we continue to discover more about life on Earth.

   隨著我們對生物體之間的關係有了更多的瞭解，並改善用於分類的標準，分類方案將發生變化。正如表面上簡單的問題「我們應該把藻類歸入哪個界？」所顯示的那般，教科書中的分類法是暫定的，隨著我們繼續發現更多關於地球上生命的事實，會對其進行修訂。