For a large number of problems there will be some animal of choice or a few such animals on which it can be most conveniently studied.
Notes from Wikiquote
See Krogh Principle | Famously quoted by an important microbiologist in: Krebs H. A. (1975). The August Krogh Principle: "For many problems there is an animal on which it can be most conveniently studied." Journal of Experimental Zoology 194:221–226.