( ( Their form ) ( ( ( is ( ( variable , ) ( ( ( some species ) ( having four ) ) ( ( ( ( ( fully developed ) limbs ) , ) and ) ( others ( having ( vestigial ( hind limbs ) ) ) ) ) ) ) ) only ) . ) )
(ROOT (S (NP (PRP$ Their) (NN form)) (VP (VBZ is) (NP (NP (NN variable)) (, ,) (NP (NP (NP (DT some) (NN species)) (VP (VBG having) (NP (CD four)))) (NP (NP (ADJP (RB fully) (VBN developed)) (NNS limbs)) (, ,) (CC and) (NP (NP (NNS others)) (VP (VBG having) (NP (JJ vestigial) (JJ hind) (NNS limbs))))))) (ADVP (RB only))) (. .)))
Their form is variable, some species having four fully developed limbs, and others having vestigial hind limbs only.
(ROOT (S (NP (NNP Birds)) (VP (VBP have) (NP (CD four) (NNS limbs))) (. .)))