Writing a paragraph with the sources properly cited can seem a tricky task at first, but the process is straightforward enough, especially when we analyze an example. Writing a sound paragraph is really just a matter of thinking clearly about a topic you have researched and transferring that thinking to the page. To illustrate, a tidy sample paragraph follows, with the sources properly documented in the author-year system. Next, the genesis of the paragraph is analyzed.
The millions of species of plants and animals on the earth have a phenomenal influence on the human species. Not only do they provide a substantial amount of our food, they are of great value in medicine and science. Over 60 percent of the purchases we make at the pharmacy contain substances that are derived from wild organisms (Myers 2008). Studies of plants and animals have led to discoveries in virtually all of the sciences, from biology and chemistry to psychology and astronomy (Wilson 2001). Furthermore, plants and animals are vital to the maintenance of our ecosystem. Their diversity and balance directly control food webs, nutrient diversity, supplies of fresh water, climate consistency, and waste disposal (Eberly 1988). Finally, many species act as barometers of our environment. The salmon, for example, is extremely sensitive to changes in the condition of the water in which it lives. Any abnormality in population or behavior of fish usually indicates some type of chemical imbalance in the water. The same is true of butterflies and their responses to the environment within prominent agricultural areas. Clearly, the millions of species of plants and animals in the world are vital to the continued thriving of the human population.
Now let’s walk through the paragraph and its use of sources. The first two sentences assert the author’s personal view about the value of the world’s species (a view shaped by his research, no doubt), which he is about to back up by using three recent sources. Next, the author cites a journal article (Myers) from which he extracted a statistic ("over 60 percent of the purchases we make at the pharmacy"). Without this source cited, the reader might believe that the author estimated loosely or simply relied on his memory for the statistic. The next source (Wilson) is cited because the paper author borrowed a general claim from a textbook by Wilson. The author was at first not sure whether to cite the source, but he wisely decided that he should because he realized that he had in fact had Wilson’s book open to a particular page and referred to it as he wrote the sentence. The next source (Eberly) is cited because the author had browsed through a whole chapter of Eberly’s book in order to compose the list in the sentence, usually using Eberly’s exact section headings from the chapter as the list members. The final examples of the salmon and the butterfly were based directly on the author’s personal experience of working at a fish hatchery for a summer, so documenting sources was not an issue. The fact that the author finds a way to tie this experiential knowledge in with his research is testimony to the fact that he is thinking as he writes the paragraph. He blends his sources, but he does not allow them to do the thinking for him. More evidence of the author’s control over his material resides in his transparent mid-paragraph transition sentence (beginning with "Furthermore"), his labeling of species as "barometers" of the environment a few sentences later, and his closing sentence, which wraps up the paragraph’s ideas neatly by making an affirmative and confident statement that backs up his topic sentence and examples.
Not every paragraph should look exactly like this, of course, but every paragraph should be written with the same kind of conscientiousness about how, when, and why the sources are cited.