Step 1: Understanding the Question
The question asks to identify the main contrast or central conflict presented in the passage. We need to look at the overall structure of the author's argument.
Step 2: Analyzing the Structure of the Passage
The passage begins by stating, "The food of hummingbirds has been a matter of much controversy." This immediately sets up a contrast. The author then outlines the two sides of this controversy:
- The Old Belief: "All the early writers down to Buffon believed that they lived solely on the nectar of flowers..." This represents the historical scientific view.
- The New Belief: "...but since that time, every close observer of their habits maintains that they feed largely... on insects." This represents the modern (at the time of writing) scientific view, which the author supports with evidence from Azara, Bullock, Waterton, and Gosse.
The entire passage is structured to contrast this old, outdated belief with the new, evidence-based understanding.
Step 3: Evaluating the Options
(A) Hummingbirds with long tails and short tails: This is a minor detail mentioned in Mr. Gosse's observation and is not the main contrast of the passage.
(B) The results of feeding...: The passage describes observations in the wild and dissections, not controlled feeding experiments.
(C) The author's opinion and Mr. Gosse's opinion: Mr. Gosse is used as supporting evidence for the author's opinion. There is no contrast between them; they are in agreement.
(D) Hummingbirds that eat flower nectar and... that eat insects: The passage argues that the same hummingbirds eat both (or primarily insects), not that there are two different types of hummingbirds with different diets. The contrast is about the belief regarding their diet.
(E) The beliefs of historical scientists...and...scientists of the author's time: This accurately describes the core structure of the argument. The author contrasts the "early writers" with the "close observers since that time."
Step 4: Final Answer
The central theme of the passage is the evolution of scientific thought regarding the hummingbird's diet, contrasting the historical belief (nectar only) with the contemporary belief (largely insects).