ely an illustration that external physical agents had not undergone
any well-marked variation in the time with which he was concerned.

[Sidenote: Nature of variation of physical conditions.] What is here
meant by variation in physical forces or condition is not any intrinsic
change in their nature, but the varied manner in which they may work by
interfering with one another, or experiencing declines of intensity.
From the fact that we may read in the fixed stars, through the
progressive motion of light, the history of a million of past years, we
may be sure that the forces of nature have undergone no intrinsic
change; that light was propagated at the same rate, was capable of
producing the same optical and chemical effects, and varied in its
intensity by distance as it does now; that heat determined corporeal
magnitudes. These are things that in their nature are absolutely
unchangeable. Always, as now, the freezing of water, and its boiling
under a given pressure, must have been the same; there must have been a
thermometric zero of life and an upward limit, no animal process ever
going on below 32 deg. Fahrenheit or above 212 deg. Fahrenheit.

[Sidenote: Effect thereof on organisms.] But out of this invariability
of natural causes variations in their condition of action arise, and it
is these that affect organic forms. Of such forms, some become at length
incapable of maintaining themselves in the slow progress of change;
others acclimatize, or accommodate, or suit themselves thereto by
undergoing modifications, and this was at last discerned to be the true
explanation of extinctions and appearances, events taking place very
slowly in untold periods of time, and rather by imperceptible degrees
than by a sudden catastrophe or crisis.

[Sidenote: Transmutation of species.] The doctrine of the transmutation
of species has met with no little resistance. They who have refused to
receive it as one of the truths of Nature have perhaps not given full
weight to physiological evidence. When they ask, Has any one ever
witnessed such an event as the transmutation of one species into
another? has any experimenter ever accomplished it by artificial means?
they do not take a due account of time. In the Fables it is related that
when the flowers were one evening conversing, "Our gardener," said the
rose to the lily, "will live for ever. I have not seen any change in
him. The tulip, who died yesterday, told me that she had remarked th