74 The Different Modes of Origin of nciv Species. 
usually ranged as elementary species of larger species, 
but sometimes regarded by the best authorities as "good" 
species. The group most closely related to our experi- 
mental series is that of Oenothera bicnnis or the subgenus 
Onagra; 1 more remotely connected are the groups of 
Hicracium, Rosa, etc., or of Draba vcrnci, Viola tricolor 
etc. Such groups appear to us as the relics of past periods 
of mutation. The new forms which arise from such 
periodical mutations are to be regarded as homonomous 
subdivisions of the older species or as elementary species. 
It is natural in such periods not only that new specific 
characters should arise, but that old established latent 
ones should reappear more easily than at other times ; 
and among the mutations of Oenothera Lamarckiana our 
O. nanclla is undoubtedly analogous to typical horticul- 
tural dwarf varieties, and 0. laei'i folia to those systematic 
varieties which arise by the loss of a character. 
These latter, however, and similar retro- and degres- 
sive changes are an entirely different matter. The essen- 
tial condition for their production is always present, and 
all that is needed is the external stimulus to induce the 
mutation. This, it appears, need not occur periodically, 
nor affect several characters at the same time. New horti- 
cultural varieties appear at irregular intervals of time, 
and here and there in the area of cultivation of the spe- 
cies. But it is equally certain that we have to do in such 
such cases exclusively, or almost exclusively, with retro- 
gressive and degressive changes. 2 Analogy and paral- 
lelism are universal, and their effects sometimes go so 
far that the characters of the species fall into the back- 
ground. Double flowers look so much alike that one 
'See Vol. I, p. 439; and 31, p. 490. 
2 1 am excluding from this consideraton the effects of crossing. 
