. The biology of the amphibia. Amphibians. THE MODE OF LIFE HISTORY 59 not, however, develop the balancer or elongate digits of primitive pond salamanders and hence is merely a "plethodontid larva" with a low dorsal fin. It has been suggested that the life history of Hemidactylium may be explained by assuming that Plethodon was originally aquatic or at least laid its eggs in the manner of Eurycea and that the life history of Hemidactylium is a retention of this primitive condition. It seems more likely, however, after a consideration of the yolk reduction of Salamandra atra and some.
. The biology of the amphibia. Amphibians. THE MODE OF LIFE HISTORY 59 not, however, develop the balancer or elongate digits of primitive pond salamanders and hence is merely a "plethodontid larva" with a low dorsal fin. It has been suggested that the life history of Hemidactylium may be explained by assuming that Plethodon was originally aquatic or at least laid its eggs in the manner of Eurycea and that the life history of Hemidactylium is a retention of this primitive condition. It seems more likely, however, after a consideration of the yolk reduction of Salamandra atra and some. Fig. 19.—The larva of Hemidactylium scutatum. Marsupial Frogs, that the aquatic period in the life of Hemi- dactylium is a derived condition induced by the reduction and early absorption of the yolk in the embryos. The European cave salamander Hydromantes shows in its osteology an affinity both to Eurycea and to Plethodon. It retains the eggs in the oviduct and gives birth to fully transformed young. The same habit is found in the neotropical salamander Oedipus structurally allied to Hydromantes. This would seem to afford evidence that these genera are closely related. Many tropical frogs pass their whole lives on land. There are oppor- tunities for laying their eggs in water but the dominant groups are those which have given up this primitive habit. Similarly, Oedipus, which has freed itself from the necessity of returning to water during the breeding season, represents a highly successful stock. From the data available, it cannot be determined whether Oedipus evolved from Plethodon or from Eurycea. At least we may conclude that the ovoviviparity of Hydromantes and Oedipus is further evidence of the close affinity of these two genera. Salientia.—Turning to the frogs and toads, there are far more genera to consider and their life histories are less known than those of urodeles. Still, there is abundant evidence that their mode of life history has usually changed gradually
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