Final answer:
The question refers to a hypothetical animal with extremely fast breeding rates, but realistically, environmental factors typically limit such growth. Fleas are an example of an organism with a high reproductive rate, yet reaching over 1 million descendants in 18 months is exaggerated.
Step-by-step explanation:
The animal known to breed so quickly that just two of them can have more than 1 million descendants in 18 months is likely a species with a very high reproductive rate, such as certain insects or small mammals like rodents. However, this question seems to be a hypothetical exaggeration, as most animal populations are kept in check by environmental factors, predation, and disease. For example, while fleas reproduce at a faster rate than larger mammals like elephants, environmental constraints would prevent them from reaching a population of over 1 million in such a short timeframe. The subject touches on concepts of reproductive strategies and population dynamics, where some species reproduce quickly and in large numbers with little parental care, while others have fewer offspring with extensive care.