■194 University of California Publications in Geology [Vol.7 



slow change through continuous variations. On the other hand, 

 the differences in tooth fornnila and in the number of caudal 

 vertebrae may indicate meristie variation, integral, and so dis- 

 continuous. 



According to Darwin : "A strictly terrestrial animal, by occa- 

 sionally hunting for food in shallow water, then in streams or 

 lakes, might at last be converted into an animal so thoroughly 

 aquatic as to brave the open ocean." 40 We look at the highly 

 specialized sirenian and cetacean, and it is difficult to realize 

 how its present form could have been reached through gradual 

 transition from a primitive land-living to a highly specialized 

 aquatic form. Darwin 41 realized, of course, the difficulties for 

 his own natural selection hypothesis involved. Nevertheless he 

 believed that, although the end product of the evolutionary pro- 

 cess might be widely different from its parent land form, still 

 each stage in the transition of the organs involved might be useful 

 to its possessor. This study of the sea-otter surely does not con- 

 flict with at least this much of Darwin's view. 



It may further be stated that many of the facts as brought 

 out in this study might indicate that the evolution has resulted 

 from a combination of methods, and that Whitman's suggestion 42 

 as to the probabilities of the reconciliation of natural selection, 

 orthogenesis and mutation may be as near the truth as it is 

 possible to come at this time. 



The imrjerfection of the geological record will probably always 

 be an obstacle to the tracing through of all stages of aquatic 

 adaptation in a single mammalian phylum. We may consider 

 ourselves fortunate if we are able to approximate these stages in 

 different groups. This we seem to be able, in a measure, to do. 

 Within the Mustelidae we have examples of animals entirely ter- 

 restrial (as Gulo, Martes, and Mephitis) ; one genus which lives a 

 terrestrial and river life (Lutra) ; and one which is almost ex- 

 clusively marine (Latax). In the reptilian order Ichthyosauria, 



■40 Darwin, C, Origin of species (ed. 6 reprint; New York, Peter 

 Eckler, 1859), p. 200. 



*i Darwin, C, ibid., p. 155. 



±2 Whitman, C. O., ' ' The problem of the origin of species. ' ' Congress of 

 Arts and Science, Universal Exposition, St. Louis (Boston and New 

 York, Houghton, Mifflin & Co.), vol. 5 (1904), p. 44. 



