Final answer:
The fertile soil in the middle colonies contributed to their unique development before 1754 by promoting a thriving farming economy.
Step-by-step explanation:
One specific environmental feature of the middle colonies that contributed to the region's unique development before 1754 was the fertile soil.
The middle colonies, including Pennsylvania, had rich and fertile land that was ideal for agriculture.
This led to the development of a thriving farming economy in the region, with crops such as wheat, corn, and barley being grown.
The fertile soil attracted many settlers, including farmers and agricultural entrepreneurs, who played a significant role in the economic growth and success of the middle colonies.