570 Species According to the Theory of Mutation. 



more nunieroiis. Every more highly organized being has, 

 as a rule, more of them than its ancestors of long ago had. 

 In applying this principle to the doctrine of elementary 

 characters we see at once that the number of these units 

 must increase with increasing differentiation ; or, con- 

 versely, that the degree of differentiation is ultimately 

 determined by the number of elementary characters. 

 Whenever a new unit is added to those already existing, 

 dift'erentiation advances a step forward. If it were pos- 

 sible to count the units we should have a measure of the 

 degree of differentiation of all organisms. 



Obviously the individual steps are only small ones, 

 at the present time at least ; and any single one of them 

 can hardlv eft'ect a noticeable increase in dift'erentiation. 

 At any rate we have at present no means of so exactly 

 measuring the degree of differentiation, since we cannot 

 estimate the possible influence of a single unit on a com- 

 plex built up of thousands of them. Only groups of units 

 produce clear and obvious differences in the degree of 

 organization; but within the limits of a small genus or 

 of a multiform collective species the several types seem 

 to us to be almost always equivalent. 



The individual steps into which, according to tliis 

 view, the process of gradual diff'erentiation can l^e ana- 

 lyzed, we propose to call mutations ; and since they con- 

 stitute an advance, progressive mutations. Each of them 

 contributes a new character to the complex of hereditary 

 qualities already present. 



Such a new character need not, however, become vis- 

 ible as soon as it arises, since we are not dealing solely 

 with external qualities but with the internal factors to 

 which they owe their appearance. Even as the germ 

 contains large numbers of qualities awaiting development, 



