418 ' ROSS ON EVOLUTION. 



Epoch, and it is probable that such highest type was always naked, 

 or nearly so as at present, but possessed teeth, pavement or plate 

 like at first, but gradually acquiring the more differentiated forms 

 since the later Silurian Epochs. Now if this were so, it satisfac- 

 torily accounts for the fact that previous to the time of the Fishes 

 with an exo-skeleton in part at least osseous, no animals so far as 

 known, at all approaching to homological symmetry in type, have 

 left recognizable remains ; and that while the early Ichthiopsidians 

 (in their grade) of remarkably homological symmetry, are well 

 i-epresented, the earlier (and more homologically symmetrical) 

 representatives of the various Orders of the 8auropsidia, and of the 

 Monotremata aud Marsupialia, and of the true Mammalia, until 

 the beginning of the Tertiary Period, are so very sparsely and 

 imperfectly known to us, is doubtless due to the fact that these last 

 inhabit dry land and could only be preserved when some accident 

 hurried their remains in][strata of such a character as would preserve 

 them, so that not one of them would be preserved for every thou- 

 sand that would be preserved of the marine Species. It must also 

 be remembered that they were probably much more limited as to 

 habitat and numbers. Doubtless similar reasons account for the 

 comparative scarcity of fossil remains of Quadrumana, known to 

 have existed throughout the Tertiary and Post-Tertiary, and of 

 Man, known to have existed in Britain before the last Glacial 

 Epoch there, and in France during the Epoch characterized by the 

 existence of the cave bear. It should be well understood that Man 

 differs physically in no way from the other Mammals, except that 

 he is more advanced and is the central and only completely sym- 

 metrical existing type. 



With regard to the other Orders of Mollusks, I will only 

 remark that all the marine types are, at an early period of their 

 development, free swimmers and possessed of functional eyes, 

 although many afterward become sessile, and many blind before 

 they reach maturity. And if the land Species do not so apparently 

 exhibit this phase of development, it is because they pass through 

 the corresponding transformations before leaving the Qgg. 



The Pieropoda which swarm about the great banks of floating 

 scawSfd of the mid- Atlantic, and form in the open seas of the 



