The Wallace's line is an imaginary line that separates the flora and fauna between Southeast Asia and Australia. It was named after the British naturalist, Alfred Russel Wallace, who first identified this distinct boundary in the 19th century. This line is significant because it marks a transition zone where the species distribution changes abruptly.

Geographical Context:
The Wallace's line runs through the Indonesian archipelago, specifically between Bali and Lombok, and then continues northeastwards through the Makassar Strait and Borneo. It extends further east, separating Sulawesi (Celebes) from the Philippines and Halmahera. The line continues southwards, dividing the islands of Timor and Flores, and then reaches the easternmost part of Indonesia.

Reasons for the Separation:
There are several factors that contribute to the separation of flora and fauna across the Wallace's line:

1. Continental Drift: Millions of years ago, the Australian continent was connected to Southeast Asia as part of a supercontinent called Gondwana. Over time, tectonic plates shifted, causing the separation of these landmasses. This resulted in distinct evolutionary paths for the flora and fauna on either side of the Wallace's line.

2. Oceanic Barriers: The deep waters of the Lombok Strait and the Makassar Strait acted as barriers to the dispersal of many species. These narrow and deep channels prevented the movement of land-dwelling animals and restricted the exchange of genetic material between Southeast Asia and Australia.

3. Different Geological History: Southeast Asia has a complex geological history with diverse landforms, such as mountains and plateaus, which have influenced species distribution. On the other hand, Australia has a relatively stable geological history, resulting in unique ecosystems and species adaptations.

Flora and Fauna Distribution:
The Wallace's line serves as a demarcation for distinct biogeographic realms:

1. Sundaland: This realm includes Southeast Asia and is characterized by tropical rainforests, diverse plant species, and a wide range of animal life, including tigers, elephants, and orangutans.

2. Wallacea: This realm lies between the Wallace's line and the Weber Line. It is a transitional zone with a mix of species from both Southeast Asia and Australia. It has a unique combination of flora and fauna, including marsupials, cockatoos, and various reptiles.

3. Australia: This realm is known for its distinctive flora and fauna, including eucalyptus trees, kangaroos, and koalas. It is characterized by arid landscapes, adapted plants, and unique animal adaptations.

Conclusion:
The Wallace's line is a significant natural boundary that separates the flora and fauna of Southeast Asia and Australia. It highlights the impact of geological history and oceanic barriers on species distribution. Understanding the Wallace's line helps scientists study the evolutionary processes and biodiversity of these regions, contributing to our knowledge of biogeography.