The Origin of Linaria Vulgaris Pcloria. 219 
plants. Lastly the mutation in Linaria does not appeal- 
along with others in space and time, but occasionally, 
and scattered perhaps over the whole area of the parent 
form and probably over the whole period of the life of 
this race. 
The mutations of Ocnotlicra Lamarckiana necessi- 
tated the assumption of a definite premutation, but the 
origin of the Pcloria is obviously a phenomenon of a 
different kind. 
Pcloria is often regarded as an instance of atavism. 1 
The correctness of this interpretation obviously depends 
primarily on whether this term is used in a narrow or a 
broad sense. Atavism is a reversion to ancestral char- 
acters ; in the narrow sense to the complete type of par- 
ticular ancestors, in the wider it refers only to single 
characters. But it is clear that the spurs which form 
a distinctive character of the genus Linaria must be older 
than the species L. vulgaris, which cannot therefore have 
had ancestors without the spur but with the other char- 
acters of the species ; so that L. vulgaris ancctaria can 
occupy no place in the series of ancestors. The sym- 
metry is ever so much older and L. vulgaris with regular 
flowers has certainly never existed amongst the ancestors 
of the common toadflax. Moreover the sterilitv of the 

peloric plants does not favor such a view. 
If the Pcloria must be regarded as atavistic, this view 
can mean no more than the assertion that it has arisen 
by the loss or latency of a character of the common 
Linaria. Therefore we are concerned here with a retro- 
gressive mutation, and the question arises, how far the 
differences between this case and the progressive muta- 
tions with which we have become familiar in Oenothera 
1 See L. JOST, Biolog. CentralbL, 1899. p. 149. 
