Tarzan of the Apes: enough sequels?

Edgar Rice Burroughs wrote the book Tarzan of the Apes, it was first published in 1912 in the pulp magazine All-Story Magazine, the book version was later published in 1914. The book and character was so popular that he wrote almost two dozen sequels later into the 1940's. The question is though how well were these sequels. The majority of the works were written by Edgar himself, some by other authors and a number where they finished off what a piece that Edgar started.
Here is the list of all of the Tarzan books:

Tarzan of the Apes (1912)

The Return of Tarzan (1913)

The Beasts of Tarzan (1914)

The Son of Tarzan (1914)

Tarzan and the Jewels of Opar (1916)

Jungle Tales of Tarzan (1919) (12 short stories)

Tarzan the Untamed (1920)

Tarzan the Terrible (1921)

Tarzan and the Golden Lion (1922/23)

Tarzan and the Ant Men (1924)

Tarzan, Lord of the Jungle (1927/28)

Tarzan and the Lost Empire (1928)

Tarzan at the Earth's Core (1929)

Tarzan the Invincible (1930/31)

Tarzan Triumphant (1931)

Tarzan and the City of Gold (1932)

Tarzan and the Lion Man (1933/34)

Tarzan and the Leopard Men (1935)

Tarzan's Quest (1935/36)

Tarzan and the Forbidden City (1938)

Tarzan the Magnificent (1939)

Tarzan and the Foreign Legion (1947)

Tarzan and the Madman (1964)

Tarzan and the Castaways (1965) (3 short stories)

from the critics point of view the first book Tarzan of the Apes had the most success, but his large number of sequels have had some criticism. Weather it is because none of them can be the original as there is not the lovable story of a child with dead parents who ends up being taken in by a ape who just lost her own child. As she takes and raises the human child as her own, the sequels do not have this element as of course Tarzan is a adult and has had the events of the first book already happen to him. Is this what the others are missing or is that the fact the critics are just simply looking for something too good as they are basing everything off of the original.