'Reptilia'--the class which exhibits the largest proportion of entirely
extinct forms of any one type,--that of the 'Crocodilia', has persisted
from at least the commencement of the Mesozoic epoch up to the present
time with so much constancy, that the amount of change which it exhibits
may fairly, in relation to the time which has elapsed, be called
insignificant. And the imperfect knowledge we have of the ancient
mammalian population of our earth leads to the belief that certain of
its types, such as that of the 'Marsupialia', have persisted with
correspondingly little change through a similar range of time.

Thus it would appear to be demonstrable, that, notwithstanding the great
change which is exhibited by the animal population of the world as a
whole, certain types have persisted comparatively without alteration,
and the question arises, What bearing have such facts as these on our
notions of the history of life through geological time? The answer to
this question would seem to depend on the view we take respecting the
origin of species in general. If we assume that every species of
animal and of plant was formed by a distinct act of creative power, and
if the species which have incessantly succeeded one another were placed
upon the globe by these separate acts, then the existence of persistent
types is simply an unintelligible irregularity. Such assumption,
however, is as unsupported by tradition or by Revelation as it is
opposed by the analogy of the rest of the operations of nature; and
those who imagine that, by adopting any such hypothesis, they are
strengthening the hands of the advocates of the letter of the Mosaic
account, are simply mistaken. If, on the other hand, we adopt that
hypothesis to which alone the study of physiology lends any
support--that hypothesis which, having struggled beyond the reach of
those fatal supporters, the Telliameds and Vestigiarians, who so nearly
caused its suffocation by wind in early infancy, is now winning at