But, so far as we know, the first Ungulates made their appearance upon earth quite as soon as did any other mammals from which they could possibly have sprung. _Phenacodus_, is in fact described as,[296] "The most primitive Eocene mammal yet discovered." He appears in the Lower Tertiary; while the Secondary and Mesozoic rocks beneath,--the whole period covered by which would be none too long for the evolution of Tertiary mammals generally,--are practically devoid of mammalian remains altogether, exhibiting only a few small marsupials, from which we can no more suppose _Phenacodus_ and the huge and various beasts who were his Eocene contemporaries to have developed, than from opossums the size of shrew-mice. It also complicates matters not a little to find that when placental mammals first show themselves all over the world at the beginning of the Eocene,--while this highly specialized order of the Ungulates seems to have been much the most numerous, it had a host of contemporaries, of extreme diversities of structure:--as for instance Unguiculates (or clawed animals) allied to the Hyena and the Fox, Rodents (gnawing animals) akin to the Squirrel, as well as Whales and Bats. Of the Cetaceans, Sir J. W. Dawson tells us:[297] The oldest of the whales are in their dentition more perfect than any of their successors, since their teeth are each implanted by two roots, and have serrated crowns, like those of the seals. The great Eocene whales of the South Atlantic (_Zeuglodon_) which have these characters, attained the length of seventy feet, and are undoubtedly the first of the whales in rank as well as in time. This is perhaps one of the most difficult facts to explain on the theory of Evolution.... "We may question," says Gaudry,[298] "these strange and gigantic sovereigns of the Tertiary oceans as to their progenitors--they leave us without reply." ... Their silence is the more significant as one can scarcely suppose these animals to have been nurtured in any limited or secluded space in the early stages of their development. The Bats, as is obvious, would require quite as much transformation from the generalized mammalian type as the Whales themselves, though in quite another direction. But they appear with their wings fully developed, in the Eocene, in both Hemispheres. Gaudry thinks [writes Sir J. W. Dawson][299] that it is "natural to suppose" that there must have been species existing previously with shorter fingers[300] and rudimentary wings; but there are no facts to support this supposition, which is the more questionable since the supposed rudimentary wings would be useless, and perhaps harmful to their possessors. Besides, if from the Eocene to the present, the Bats have remained the same, how long would it take to develop an animal with ordinary feet, like those of a shrew, into a bat?