II. THE RANGE OE VALIDITY OF THE DOC- 
TRIXE OF MUTATION. 
6. THE SIGNIFICANCE OF THE AVAILABLE EVIDENCE. 
L'nlike the prevailing form of the theory of selection, 
the doctrine of mutation lays stress on sudden or dis- 
continuous changes, and regards only these as active in 
the formation of species. The Darwinian form of the 
theory of selection regards both these and fluctuating 
variations as operative in the origin of new forms, whilst 
WALLACE favors the other extreme, according to which 
all formation of species goes by a slow and gradual 
process of change. 
The two schools of thought naturally adopt different 
attitudes towards the doctrine of mutation. It is at once 
rejected by WALLACE'S adherents, whilst those who in- 
cline to DARWIN'S own form of the theory are less un- 
reservedly inimical ; many of them have even greeted it 
with open arms. 
Inasmuch as these two lines of thought have been 
clearly manifested in the critiques which have been pub- 
lished of the first volume of this work, I propose to dis- 
cuss them briefly here, in order to point out the funda- 
mental questions which are involved in this controversy. 
The extreme opponents of my theory maintain that 
there are no mutations at all ; Natnra nou facit saltns, 
they say. What I have described as discontinuous changes, 
