A topic sentence at the beginning states the unifying idea of the paragraph as the first sentence and follows this statement with support.
The West Texas wolverillo has peculiar eating habits. Although its usual meals include old abandoned bathtubs and plumbing, it has been known to gnaw on horseshoes still on the horse, even with the horse in full gallop. For purposes of dental hygiene, it chews on porcelain and china either as an appetizer or for dessert, during and after which it makes growling noises.
In the paragraph above, the topic sentence is The West Texas wolverillo has peculiar eating habits. The steps involved in is the eating habits of wolverillo, it shows the unique food diet of wolverillo. Note that, although the paragraph draws to a logical conclusion, the topic sentence is not repeated in the end position.
Unity – Topic Sentence in the Middle .
A topic sentence in the middle enables writers to build up to a point and then expand their ideas.
Example: The Texas plains have provided a unique habitat for various desert creatures. These animals have worked together in a constant struggle to create a sturdier stock. The most ingenious of these Darwinian attempts is the West Texas wolverillo, a cross between a wolverine and an armadillo. There has been much speculation over the years about the origins of this creature. Most agree that the initial breed occurred as a result of the accidental isolation of two parent animals by some geological upheaval.
In the above paragraph, the topic sentence is There has...