3° 



THE QUARTERLY REVIEW OF BIOLOGY 



sentatives are often quite unlike the 

 original stock from which they have de- 

 scended. 



Whales at one time, geologically speak- 

 ing, were land mammals and, although 

 highly modified in some respects, they 

 still retain all the typical mammalian 

 features . Like land mammals they suckle 

 their young and retain vestiges of their 

 former coating of hair. With the pas- 

 sage of time their fore limbs have been 

 modified to function as pectoral flippers, 

 and their tail has been provided with 

 caudal flukes to function as an organ for 

 propulsion. No traces of hind limbs 

 have been found in any of the living 

 toothed whales with the exception of the 

 sperm whale (Physeter catodon), but never- 

 theless we are fairly certain that these 

 were present in the progenitors of the 

 Cetacea, since the whalebone whales 

 have one or two vestigial limb bones 

 buried deep in the flesh of the pelvic 

 region. Much more profound changes 

 have occurred in the "soft parts" and in 

 the physiological adjustments of their 

 organs to an aquatic life. The bodily 

 organization of living whales shows us 

 what has been accomplished in the way 

 of adaption, but it affords no proof of 

 how this has come about, and for evidence 

 of this sort we must look to the record in 

 the rocks. The skeleton is the frame- 

 work upon which the muscles and organs 

 are attached, and by studying these "hard 

 parts" and endocranial casts one can infer 

 some of the changes which were taking 

 place in the so-called "soft parts." 

 Porpoises are found in both fresh and salt 

 water, while the whalebone whales are 

 almost exclusively pelagic. Both of these 

 two groups of living cetaceans have 

 followed different paths of anatomical 

 adjustment to fit themselves to this or that 

 manner of living. 



To show that adaptation to life in the 

 water is a complex biological transition 

 for a mammal formerly accustomed to 

 land conditions and that such an adapta- 

 tion has been clearly demonstrated by 

 collateral investigations along the lines 

 of palaeontology, anatomy, embryology, 

 and physiology is the object of this 

 resume. From the very nature of the 

 evidence it is not surprising that there 

 should be some divergence of opinion as 

 to the details of this biological process, 

 but there is little or no conflict as regards 

 the fundamental principles underlying the 

 views that are based on ascertainable facts. 



Some ten years ago the writer was 

 invited to prepare a report on the fossil 

 marine mammals as a part of a research 

 program planned by Dr. John C. Merriam 

 for a study of the extinct vertebrate faunas 

 of the Pacific Coast and Great Basin 

 Provinces of North America, and during the 

 past six years this work has been supported 

 by the Carnegie Institution of Washing- 

 ton. In the course of this investigation 

 the writer has had the opportunity to 

 study the collections of fossil and recent 

 cetaceans in the United States National 

 Museum, the American Museum of Nat- 

 ural History, the Museum of Comparative 

 Zoology, the Museum of Palaeontology 

 of the University of California, and the 

 California Academy of Sciences. In the 

 matter of illustrations the writer has had 

 the able cooperation of Mr. Sydney 

 Prentice, who has prepared practically all 

 of the line drawings used in this essay. 

 In cases where no complete skull has been 

 available the illustration is based on two 

 or more skulls which mutually supple- 

 ment one another so far as missing parts 

 are concerned, but in each instance the 

 skulls selected for this purpose are con- 

 specific and were obtained from the same 

 geological horizon. 



