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I have tried to show that the best and strictest conception of varieties limits them to those forms that have probably originated by retrograde or degressive steps. Elementary species are assumed to have been produced in a progressive way, adding one new element to the store.
But there is still a third possibility. For a latent character may reassume its activity, return to the active state, and become once more an important part of the whole organization. This process may be designated as degressive evolution; it obviously completes the series of the general types of evolution. Advancement in general in living nature depends on progressive evolution.
Contrasted with this method of growing differentiation, are the retrogressive modifications, which simply retrace a step, and the degressive changes in which a backward step is retraced and old characters revived. No doubt both of these methods have been operative on a large scale, but they are evidently not in the line of general advancement.
The polypetalous genera Pyrola, Ledum, and Monotropa among the sympetalous heaths, are a remarkable instance of this. The whole evolution of the monocotyledons from the lowest orders of dicotyledons implies the seeming loss of cambial growth and many other qualities. Degressive evolution is not so common by far, and is not so easy to recognize, but no doubt it occurs very frequently.
Besides them the majority of the retrograde changes and some progressive steps might occur separately, each being due to some special cause. Degressive mutations, or those which arise by the return of latent qualities to activity, would of course belong with the latter group.
But it is obvious that these may be included under the same heads as the larger and older ones. For the larger movements are manifestly to be considered only as groups of smaller steps, going in the same direction. Hence we conclude, that even the smallest steps in the evolution of plants which we are able to observe, may be divided into progressive, retrogressive and degressive ones.
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