DEVELOPMENT OE THE SKULL OF THE COMMON EKOG. 
201 
Concluding Bemarlcs. 
The foregoing comparisons are given under tire impression that they may be of some 
immediate use to the student, although they must be given on a larger scale after several 
more types have been worked out. 
Moreover it is no little relief to the worker himself thus to be able to rise from a 
task which has used the leisure of well nigh two years ; such an expatiation is necessary 
before the renewal of a similarly persistent concentration to new work of the same kind. 
He contends, also, that the mind both of the reader and the writer will be strengthened 
as well as refreshed by a wider view, and that each separate type will then be seen in 
the light of many other types. Indeed thus alone will it be possible to obtain broad 
views in vertebrate morphology, “ as a man conveniently placed in some eminent station 
may possibly see, at one view, all the successive parts of a gliding stream ; but he that 
sits by the water’s side, not changing his place, sees the same parts only because they 
succeed, and those that pass make way for them that follow to come under his eye.” 
I must confess to having subjected myself to this mole-like burrowing into so limited 
a territory that I may obtain fresh material for ratiocination — “ that way of attaining 
the knowledge of things, by comparing one thing with another, considering their mutual 
relations, connexions, dependencies, and so arguing out what was more doubtful and 
obscure, from what was more known and evident.” 
To have worked out one single species in this way may seem to be but like the forming 
of a single track in a primaeval forest ; yet when well cleared, so perfect is the unity of 
each subkingdom, by such a narrow path the worker is “ regularly led on through the 
labyrinths of Nature, when still new discoveries are successfully made, every further 
inquiry ending in a further prospect, and every new scene of things entertaining the 
mind with fresh delight.” Leaving for awhile the suggestive morphology of the Frog, 
it may be worth while for the palaeontologist to reflect upon the empty spaces in the 
great vertebrate circle which are darkly but really revealed by what is seen in both the 
earliest and the latest stages of the Frog. 
Territories vacant, but larger far than those now occupied by family after family, and 
order after order, have been suggested to me by my long attention to the growth of the 
skull and face in this Amphibian. - 
Empty spaces of almost indefinite extent seem, to my mind, to stretch themselves 
below the Myxinoid prototypes of the Batrachia, and above and beyond the Frogs and Toads, 
in the direction of the Mammalia. 
This last space is wholly undefined, and no light has yet penetrated its deep abyss, 
m which lie buried the fundamental Mammalian types. The lowest Mammals known 
to us, the Platypus and the Echidna, maybe fundamental to the Edentata; they are not, 
they cannot be, to the Marsupials, the Insectivora, and the Xlodentia. 
Between the Monotremes and the Batrachia we certainly have the Sauropsicla — Bep- 
tiles, and Birds ; but I am bold to say that no Sauropsidan lies in a direct line between, 
forms any part of a phylum which should connect together, the nobler Amphibian forms 
mdccclxxi. 2 E 
