578 Species According to the Theory of Mutation. 



same theoretical conception of the nature of elementary 

 characters. This conception may in the present state of 

 our knowledge be most conveniently formulated as fol- 

 lows : 



Forms which have arisen by retrogressive and de- 

 gressive mutation follow Mendel's law, when crossed 

 with their ancestors; whilst forms which have arisen by- 

 progressive mutation behave uni-sexually. 



§ 3. THE THEORETICAL DISTINCTION BETWEEN SPE- 

 CIES AND VARIETIES. 



The idea of a fundamental difference between ancient 

 and recent characters runs like a scarlet thread through 

 ihe whole history of systematic biology. The nature and 

 limits of this assumed difference have often furnished 

 problems which the greatest investigators in this field 

 have attempted to solve; and the answer has been a 

 different one according to the information available at 

 the time when it was attempted. From the transmuta- 

 tionists up to Nageli's well-known distinction ]:>etween 

 organic and adaptive characters there has been a long 

 series of attempts to deal with these questions. 



In ancient times the matter was easily settled by in- 

 voking supernatural causes. The higher systematic cliar- 

 acters were assumed to have arisen by creation ; the later 

 ones by natural means ; but in practice even this view 

 led to confusion, because some authors regarded the 

 genera, others the collective species, and yet others the 

 constant elementary forms as the units which had been 

 created. 



Our discussions have led us also along several differ- 

 ent lines, to the conviction that, as a matter of fact, there 



