Progression, Retrogression and Degression. 75 
often cannot tell, even from the best illustrations, to 
which genus or family a given instance belongs. 
I shall therefore throughout this Part attempt to 
describe the origin of horticultural varieties as exhaus- 
tively as possible. In the present state of our knowledge 
they form in my opinion the pattern of retrogressive and 
degressive formation of species ; just as the mutations 
of Oenothera were the pattern of progressive changes. 
Together they give us some idea of the main lines along 
which specific differentiation takes place in nature, at the 
present time as well as in the past. 
In conclusion : Progress on the main lines of descent 
results from the production of new characters ; but the 
extraordinary variety of forms results from the occa- 
sional disappearance of characters already existing, or 
from the activation of latent ones (retrogression, de- 
gression, atavism). 
