66 The Different Modes of Origin of neiv Species. 



tinual formation of new cliaractcrs, to increasing differ- 

 entiation. Nevertheless the great nuihiforniity of spe- 

 cies within tlie 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- 

 stances of regression ; that is, with the absence of 

 characters which are otherwise proper to the group to 

 which the species belongs. Sium and Berula have, for 

 example, simple pinnate leaves within the group of the 

 Unibelli ferae with (l(jul)ly pinnate leaves ; and tlie assump- 

 tion is that they have arisen from the latter by a sim|)le 

 loss. Similarly Primula acaulis stands in the middle of 

 a group containing the Primulas, And rosace 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 wdiole groups. For instance Delpino 

 holds, as is well known, that the Monocotyledons have 

 arisen from the lower Dicotyledons by the loss of a wdiole 

 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 tlie 

 latter view, derived largely from the great varietv 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 



