REVIEWS—ON THE ORIGIN OF SPECIES. 395 
Accepting this as a clear and accurate summary, we shall find that 
the poits requiring to be carefully examined are the proper meaning 
and natural limits of Atavism and Variability, and we must recollect 
that the existence of these tendencies is equally admitted by both 
parties. The question is whether, m connection with the external 
conditions of existence, they are fully sufficient to account for all the 
phenomena of species, genera and higher groups amongst organized 
bodies, or whether the modifications they produce are subservient to 
certain determinate inherent tendencies of development, descending 
from the first created organisms and constituting the great plan of 
creation which, as we learn to interpret it, we express by the king- 
doms, sub-kingdoms, classes, tribes, genera, and species. Now this 
question is identical with that of the permanence or mutability of 
species, which is therefore, we apprehend, the real subject of contro- 
versy. Granting their mutability, we do not pretend to adduce more 
probable influences for their modification than those assigned by Mr. 
Darwin, and all who maintain their permanence believe the cases of var- 
lation brought forward to be confined to varieties and races, and to af- 
fect characters which are not essential to the species. It is easy to 
assume that the existence of certain structural resemblances implies a 
common origin, but such resemblances form an essential part of the 
notion of a plan of creation in which every position is occupied and in 
which the utmost variety is produced by special adaptations of various 
types. They are as well accounted for on the one scheme as on the 
-other—unless indeed we recognise in the regularity of corresponding 
variations of different types, the impossibility of what may be termed 
accidental causes of variation, such as are supposed in Darwin’s 
hypothesis, having any place. 
It seems to us beyond all reasonable question that what is absolute- 
ly required before we can admit the possibility of the transmutation 
theory, is the production of at least one clear instance of descendants 
of a common parent, having by the joint action of variability and 
atavism become so distinct in structure as to be fairly accounted 
separate species. This we do not believe to have been done. We 
know, however, that the want of any definition of a species in which 
both parties can agree, and the power of requiring indefinite periods 
of time to accomplish the supposed changes, will prevent this test 
being of much value for convincing opponents. We must therefore 
-be content with enquiring what we really know of atavism and varia- 
