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 
C3 
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 
the 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 between 
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 char- 
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 
