AQUATIC MAMMALS 



There is a vestige of the femur present and, at times, of the proximal 

 tibia. There is a small caecum. 



Modern whales are the most highly modified for an exclusively 

 aquatic existence of any mammals that have ever lived. Externally some 

 of the porpoises are, save in minor details, to all intents replicas of 

 ichthyosaurs and some sharks, furnishing perhaps the most spectacular 

 instance of convergence in the entire class of mammals. This fact is 

 proof positive that the whales of the more typical shape are close to the 

 ideal in body form for their habitat. They are excessively specialized 

 — in fact more so than any mammal, living or fossil, of which we know 

 — and their survival bespeaks eloquently of the lack of ecologic com- 

 petition which they have encountered. 



The published discussions regarding the derivation of the Cetacea 

 would fill many volumes, but most of this material is open to the same 

 objection that has been presented in the case of the pinnipeds. Per- 

 haps the majority still believes that they are descended from a hyaeno- 

 dont stock, more particularly from the adaptive creodonts (a conten- 

 tion refuted by Matthew, Gregory, Kellogg) ; but there exist those who 

 argue from numerous other angles, including their descent directly from 

 the Pro-Mammalia (Albrecht), the ungulates through the sirenians, 

 that their closest aifinities are with the pinnipeds, or the edentates, and 

 even that their ancestors never were land mammals but aquatic verte- 

 brates. Some of the most interesting evidence in argument against the 

 carnivorous ancestry of the whales has been presented by Anthony 

 (1926), comprising anatomical details that are notoriously conservative. 

 He stated that the following points are common to some of the whales 

 and certain ungulates: Lateral nasal diverticula; diverticula of the Eus- 

 tachean tubes ; communication of the pleura through the anterior medias- 

 tinum; tapetum choroideum of the eye composed of fibrous lamellae; 

 absence of os penis (some whales are said to have this) ; ovaries re- 

 sembling Hyrax; frequent persistence of Mullet's canals in males; 

 placenta adeciduate and diffused; hippomanes body present in fetal 

 membrane ; usually but one young at birth ; long period of gestation ; and 

 young large and well developed. 



Flower and Kellogg at least have gone on record in their belief that 

 the terrestrial ancestors of the whales must be sought among Cretaceous 

 remains, and this is a most plausible contention. It seems not improb- 

 able that the cetacean stock is older than that of the true carnivores or 

 of the ungulates, and therefore this ancestor was presumably a mem- 

 ber of the carnivore-insectivore stem that is believed to have been ances- 



[44] 



