944 THE STRUCTURE OF EVOLUTIONARY THEORY
only after the origin of live birth also derives no support from documented
intermediary stages. In the traditional view, the shell membrane between fetal and
maternal tissues must thin gradually, permitting an initial function of placental
organs in uptake of water and exchange of gases. Placentotrophy then evolves later
"as the placental supply of nutrients first supplements and then supplants provision
by the yolk" (p. 208). But evidence from at least 19 independent clades of
viviparous squamates indicates that all "have anatomically recognizable placentae
derived from both the chorioallantois and the yolk sac" (p. 208). Thus Blackburn
concludes, "the existence of a truly non-placental viviparous squamate has not
been documented in over a century of investigation ... The universal occurrence of
placentae in viviparous squamates is most consistent with the view that placental
organs that accomplish gas exchange and water uptake evolve simultaneously with
viviparity" (p. 209).
Similarly, no purely lecithotrophic (yolk feeding) placental squamates have
been discovered, and all viviparous forms derive at least some nutrition through the
placental organs. Thus, "available data are most consistent with the hypothesis that
incipient placentotrophy is a necessary correlate of viviparity." The three "logical"
steps of the hypothetically gradual trend become telescoped into a single structural
transition, with an evident implication of punctuational origin.
9 - 35. Punctuational change in the morphological evolution of lineages in squamate reptiles,
Ovoviviparity does not evolve by progressive and gradualistic delay of oviposition, but rather
shows marked bimodality with females either depositing eggs with embryos in their early limb
bud stages or else retaining the eggs within their body to term, and then giving birth to live
young (the right mode). The existence of a few intermediary species shows that the full sequence
proceeds by punctuational steps and not by full saltation.