664 Geological Periods of Mutation. 
small number of characters necessary for such identi- 
fication. If, in the case of single forms, we look at a diag- 
nosis of species, genus, family or order, we find only a 
small series of characters referred to. If we attempt to 
describe a higher plant as completely as possible, it be- 
comes difficult to prolong the list for over more than 
a few hundred characters; and even if we have regard 
to internal structure, 1 latent characters, and so forth, it 
is verv difficult to attain to thousands of characters. 
/ 
The significance of this difficulty is best illustrated by 
the fact that such a description of a single form would 
cover over a hundred pages of print. 
The structure of our eye is infinitely wonderful ; the 
series of intermediate stages between it and a simple 
spot of pigment is immeasurably great ; and a period of 
millions of years would be needed on the theory of selec- 
tion for the attainment of the present high degree of 
organization from those first beginnings by means of 
ordinary variability. 2 But MURPHY, BROOKS and many 
others have pointed out that these considerations do 
not necessitate the conclusion that it must have happened 
in this way. 3 On the contrary, the extraordinarily long 
time which the theory demands, leads us to suspect that 
there is some weak point in the argument. 
It is perhaps here that the theory of mutation, re- 
garded from a general point of view, manifests its great- 
est advantages over the prevailing form of the theory of 
selection. In the first part of the first volume I have 
attempted to show that it agrees with results of experi- 
1 A. GRAVIS, Reck. anat. sur les org. veget. de I'Urtica dioica, 
Mem. sav. etr. Acad. Beige, Vol. XLVII, 1884; and the same author, 
Rech. anat. et phys. sur le Tradescantia virginica, ibid., 1898. 
2 DARWIN, Origin of Species, p. 143. 
8 W. K. BROOKS, Heredity, 1883, 2d. ed., p. 283. 
