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,-^ latent characters, and so forth, it 

 is very 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.^ 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.^ 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- 



^ A. Gravis. Rcch. anaf. sur les org. veget. de I'Ui'fica dioica, 

 Mem. sav. etr. Acad. Belee. Vol. XLVII. 1884; and the same author, 

 Rech. anat. et phys. stir le Tradescantia virginica, ibid., 1898. 



^ Darwin, Origin of Species, p. 143. 



^ W. K. Brooks, Heredity, 1883, 2d. ed., p. 283. 



