358 



BIOLOGY 



mack 



From this series of observations, extended in other directions 

 by many other observers, has been developed the theory of 

 mutation. This theory is, in essence, that new characters do 

 not, as a rule, appear simply as slight diversities found between 

 different individuals of the same species, but as characters of 

 considerable extent at a single birth. New features of the race 

 are thus sudden in their origin instead of gradual, as had been 

 supposed by Darwin and also by Lamarck. According to this 

 theory there are two types of variation among organisms: 1. 

 Individual variations, spoken of above as the diversities which 



are shown between 

 different individuals, 

 and which come and 

 go in a haphazard 

 fashion, having no 

 part to play in the 

 change of the race. 

 These may be ac- 

 quired characters; at 

 all events they are 

 not impressed upon 

 the germ plasm. 2. 

 Mutations, which 

 probably start with the germ plasm; Fig. 144. These varia- 

 tions may be large or small, but whenever they appear they 

 are at once fixed in the race. Inasmuch as they are part oi 

 the germ substance, they will be handed on to the next gener- 

 ation and remain, therefore, as a permanent inheritance of the 

 race. According to the mutation theory, these sudden large 

 changes have brought about the race divergence. The theory 

 of mutation, therefore, abandons Darwin's idea of the accumu- 

 lation of the minute diversities, and replaces it with the idea 

 that the steps in evolution may be larger and may be taken sud- 

 denly. It is, of course, evident that this new conception of muta- 

 tion is perfectly consistent with Weismann's view of heredity, 



Red 



BY THE 



FIG. 144. MUTATIONS SHOWN 

 BEETLE LEPTINOTARSA 



A and C are mutants from the original form B. The 

 actual differences are greater than appears in these fig- 

 ures because of great differences in color. 

 (Tower.) 



