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tions of climate and soil, or of other conditions, are accompanied by 

 corresponding variations in the plants. But, limiting the period of 

 our observation, as above, these variations are usually found to be 

 temporary ; so that we may say, there is a standard or average type 

 for each kind, which is repeated in the individual plants as nearly as 

 internal health and external conditions will allow. This supposed 

 standard or average 1 will here express by the term ' central type.' 

 The central type will thus be nearly synonymous with the proper 

 metaphysical notion of a species, apart from its varieties. It is usu- 

 ally this same central type which is described in the specific charac- 

 ter of a plant; although occasionally authors endeavour so to form their 

 specific characters, that these shall include all varieties of the species 

 as well as the central type. 



Individual plants which differ from the central type are designated 

 " varieties." Among varieties we may include all the plants which are 

 marked by any obvious difference, ranging from the more trifling 

 variations of colour or size, to those which are so wide as to raise a 

 question whether the plants really belong to the same central type or 

 species. 



Varieties appear to be less permanent than the central types from 

 which they originate. A tendency to change again is usually observed 

 in the descendants of such varieties ; and the further change is 

 frequently in a reversed direction, or back towards the central type. 

 Among plants in a wild state, the tendency to keep or to resume the 

 central type commonly seems to be greater than the tendency to vary 

 from it. Hence there is an appearance of permanence in species, as 

 though each kind had a limit to its power of change, beyond which 

 its descendants can never pass in a direction aberrant from the central 

 type, and from which limit there is a tendency to return to that type. 



But all this, be it remembered, refers to a very restricted period in 

 the history of our globe. It is that space of time only, the events of 

 which are most clearly seen and understood by botanists. And it is 

 so very short a space, comparatively with the spaces which come into 

 the estimates of geologists, that we can scarcely deem an inappreci- 

 able change of the central type, during that short space, to be suflici- 

 ently conclusive proof against the gradual transition of species during 

 spaces of time immeasurably more extended. 



Moreover, we must avoid the straining of our fact beyond its true 

 bearings. Though the central types of certain species may have 

 remained the same during some scores or centuries of years, this one 

 fact cannot negative a possibility that there are also varieties of the 

 same or of other species which, during the same time, have gradually 



