He also recognized that the ratites were secondarily flightless. In contrast, large herbivorous flightless ratites have survived to the present. Struthioniformes contained all the ratites, such as the kiwis, emus, rheas, and cassowaries. He rejected the Ratitae-Carinatae classification that separated tinamous and ratites. This indicates that the distinctive flightless nature of ratites is the result of convergent evolution. The great spotted kiwi, as a member of the ratites, is flightless. The evolutionary history of cassowaries, as of all ratites, is not well known. They are absent in grebes and some ratites, and greatly reduced in size in penguins. Today, ratites such as the ostrich are farmed and sometimes even kept as pets. Like most other ratites, they have no uropygial gland (preen gland). Some birds (e.g., ratites) have lost the primary locomotion of flight. The smallest ratites are the five species of kiwi from New Zealand. It is the only structure found in primitive birds such as ratites. In particular, it explains the Gondwanan distribution of ratites and the Antarctic flora. Some of the earliest ratites occur in Europe.