88 THE MICROSCOPICAL NEWS. 
only necessary to escape from an enemy, they have acquired fleet- 
ness or cunning. The appendages of insects and shell-fish are 
frequently cast off ; and, as with the odontophores and the teeth of 
fishes, are being constantly renewed. But in the higher forms of 
animals there is generally but one renewal; and when the per- 
manent teeth are once in place they must suffice. The elephant 
is a notable exception to this rule; for of the six huge molars he 
gets on each jaw there is never more than one, or at best two 
partially, in place and use on each side at a time. ‘The series is 
continually in progress of formation and destruction, of shedding 
and replacement, the new tooth succeeding the departing one hori- 
zontally from behind forwards, none being displaced by vertical 
successors. Both male and female elephants have tusks. These 
are really the incisors, although in structure they are not quite 
like ordinary teeth, but consist of the modified dentine called 
ivory. This is recognised by striz proceeding in the arc of a 
circle from the circumference in opposite directions, and forming 
by their decussations curvilinear lozenges, and very much resemb- 
ling the engine-turning on a watch case. 
The teeth are not bones. Bones are developed by the meta- 
morphosis of true cartilage and ordinary connective tissue ; and in 
both cases the cells are converted into lacunz, or what Kolliker 
calls bone-cells ; but the teeth are dermal structures. ‘That struc- 
tures do arise from the dermis is apparent in the hoofs, hairs, &c., 
of animals, the carapaces of some of the crustacea, the scales of 
fishes, and so forth. Indeed, according to Gegenbaur, the ancestry 
of the whole dental system can be traced back to the placoid 
scales of fishes; and this view has been adopted by the best 
authorities. Of these scales Gegenbaur says they have “the struc- 
ture of dentine, are covered by enamel, and are continued into a 
plate formed of osseous tissue.” ‘Therefore, he calls them “dermal 
denticles.” But how do structures that originate on the external 
surface for protection eventually develop on the internal surface for 
purposes connected with digestion, and assume such different 
forms? ‘This is not difficult to understand when we remember, 
first, that the mucous membrane is only a differentiation of the 
skin; and, second, that constant variation to suit varying condi- 
tions is the order of life. As scales are formed on the skin of 
fishes we can see how easily they might extend beyond the border 
line which divides the external from the internal coating and spread 
over the mouth. Now, once begin the development of hard 
substances in the mouth, if we but allow the inconceivable ages 
which the comparative anatomist tells us have been required for 
the development of the higher animals, and the long succession of 
ever-changing environment which all animals are constantly passing 
through, it is not difficult to imagine how the primitive forms 
