Discussion

In his classification of lizards, Camp (1923) attempts to evaluate palacotelic characters, that is, characters reliable as an indication of relative primitiveness. He emphasises that a species exhibiting a few characters of high antiquity may be considered more ancient than one having many characters of lesser palaeotelic value (comparative rank), and takes the important step of assigning comparative rank to some 34 selected characters. This is necessitated by the fact that high specialisation in a group may obscure an archaic position that can only be traced by recognition of satisfactorily primitive characters. Camp's list for lizards generally is accordingly headed by such features as (a) three complete branchial arches; (b) vertebrae amphicoelous; and (c) two complete skull arches.

Underwood (1954) draws extensively on the resources of his studies on reptilian eye structure to propound a classification of geckos. His proposed classification implies that the amphicoelous vertebral condition is secondary to the procoelous condition, and this implication he initially accepted. Underwood has recently indicated that he is withdrawing this view because of palaeontological evidence. A considerable radiation of definitive lizards occurred in the Triassic, and all of these are amphicoelous. The rise of definite lizards long antedated the fixation of the procoelous condition.

The position of any group of lizards, or of members within a group, can only be ascertained after a careful survey of all the available data. Where there is a clash as in this instance between eye and vertebral structure in geckos, then Camp's procedure of assigning comparative value to these characters must surely be adopted.

A similar situation arises when considering the morphological status of the New Zealand geckos. On the one hand they are osteologically primitive, and on the other they are the only geckos in the world known to be ovoviviparous.

H. Claire Weekes (1935), in reviewing placentation among Australian scincs and snakes, suggests that cold is the most likely external factor associated with high altitudes that may influence either directly or indirectly the adoption of ovoviviparity. In referring to the ovoviviparity of the New Zealand geckos she observes that the New Zealand climate, approaching that of the British Isles, is the coldest in which geckos occur. Weekes concludes from her study that placentation among reptiles has arisen independently many times in the course of evolution, and that the phenomenon of parallel development of similar types of placentas is common.

Mary M. M. Boyd (1942), commenting on Weekes' findings, concludes that since oviparous and ovoviviparous species occur in the same genus, placentation must have arisen fairly recently, after the divergence of the species. She states that the simple type of placenta in Hoplodactylus is according to expectation “since viviparity must have been assumed after the divergence of the species”.