VI 
INTRODUCTION. 
coveries follow one another in this almost boundless field of science ; and when to this is 
added the positive evidences of variation under domestication, the implied evidence to he 
found in the coexistence of infinitely minute gradations of form in every branch of the animal 
and vegetable kingdoms, and the hitherto insurmountable difficulties which have surrounded 
systematists, and up to the present time have rendered all definitions of the usual expiessions 
of classification unsatisfactory, a good j/rima facie case has been made out for accepting the 
proposition that all known forms of living organisms are capable of spontaneous variation 
which may become hereditary. But that this capability is dormant at most times, that its 
exercise is an exception to the ordinary laws of nature, and that it is, in the majority of 
instances, called into play by a change in the outward circumstances involving a wide departure 
from the ordinary conditions of life, there are many equally (if not more) clear indications 
throughout Nature. In other words, hereditary permanence seems to be a principle of the 
law of generation, while enforced changes of condition call into play a power of variation, 
dormant while those conditions remain unchanged. 
The evidence furnished by geology seems to compel us either to admit the hypothesis of 
modified descent, or hold to the idea of successive creations ; though neither of them can in 
the present state of the science be absolutely proved from it. That hereditary variation is 
possible, and exists in the manner above stated, we fully admit ; but the incalculably small 
effects that it has produced in recent periods warrant grave doubts as to its being the chief, 
much less the sole, agent in the origin of species : if it be the correct hypothesis, the geological 
record ought to show a continually decreasing ratio in the number and diversity of forms of 
life as we look backwards through time, culminating of course in the single primordial 
form. On the other hand, if diversified creation be the true solution of the problem, 
the receding geological strata should exhibit fewer and fewer intermediate forms, and con- 
tinued isolation of types, till the original parents alone were left of each group of organisms, 
whether at present existing, or that have existed and are now extinct, widely diffeiing from 
each other at first in all probability, though their offspring, from numerous complex causes, 
have varied considerably in adapting themselves to the changing conditions of life to which 
they have in the course of time been exposed. Geologists must settle which of the two pro- 
positions is best borne out by the facts, though we fear that these are not sufficiently decisive 
as yet to afford satisfactory proofs. The incompleteness of the record from lack of investiga- 
tion, and still more so from the very inconsiderable fraction of the whole organic remains of 
past ages that has been thus preserved, will render the actual demonstration of the truth or 
falsity of any theory impracticable : indeed the evidence conveyed in the obtiusive appearance 
of new and unexpected genera at late periods of the Earth s histoiy is not more damaging 
to the one hypothesis, than the absence of heavily charged sub-Silurian fossilifeious 
deposits is to the other ; and even now the forms found in the oldest strata exhibit great 
diversity. 
With regard to the origin of life, the laws of generation do not help us, for they start with 
presupposing its existence ; and if the inquiry is to embrace the origin of inorganic as well 
as organic matter, we meet ultimately an impassable barrier — a scientific impossibility the 
