66 The Different Modes of Origin of new Species. 
tinual formation of new characters, to increasing differ- 
entiation. Nevertheless the great multiformity of spe- 
cies within the orders and families is only in part due to 
this progressive process, but to a large extent to an in- 
finite variety of combinations of characters already exist- 
ing. This is combined in innumerable cases with in- 
o 
stances of regression ; that is, with the absence of 
characters which are otherwise proper to the group to 
which the species belongs. Sinm and Bcrula have, for 
example, simple pinnate leaves within the group of the 
Umbclli ferae with doubly pinnate leaves ; and the assump- 
tion is that they have arisen from the latter by a simple 
loss. Similarly Primula aeanlis stands in the middle of 
a group containing the Primulas, Androsace etc. with 
umbellate inflorescences, and the same inference is drawn 
as to its origin. The same is true of a host of other 
cases, and even for whole groups. For instance DELPINO 
holds, as is well known, that the Monocotyledons have 
arisen from the lower Dicotyledons by the loss of a whole 
series of characters. 
Cases such as these are spoken of as instances of 
retrogressive metamorphosis. And it is probably not 
too much to say that there are possibly more species on 
the face of the earth at present that have arisen on retro- 
gressive than on progressive lines. 
The question is often debated whether, in retrogres- 
sion, the characters absolutely disappear or only become 
invisible, or latent. There is much evidence for the 
latter view, derived largely from the great variety of 
atavistic structures (youth forms, subvariations on the 
lower internodes of lateral branches, the form of the leaf 
in suckers, the effects of parasites, anomalies, reversions 
to the ancestral form by bud-variations, etc.). Latency 
