INTENSION. 195 



8. Seleclional Intension, or Segregation and Divergence Produced by 



Independent Selection. 



That we may gain a clear apprehension of the nature and influence 

 of this principle, certain discriminations, which have not always been 

 recognized by writers on the subject, are absolutely necessary; and, 

 for the sake of avoiding misunderstandings, it is desirable that these 

 distinctions should be represented by clearly defined terms. I am 

 fully aware that many will be opposed to the introduction of new 

 terms into the treatment of a subject that has been so long and ably 

 discussed. If these discriminations were not found necessary by the 

 author of the "Origin of Species," or if the distinctions, so far as 

 recognized by himself and others, have been expressed in the language 

 of ordinary description, why should a more accurate terminology be 

 needed now? In reply it may be said that the freedom from tech- 

 nical language, which is a great advantage in a work which for the first 

 time calls the attention of the world to a vast subject, is a serious 

 defect when the exact relations of the subject come under discussion. 



In order to secure clear thinking on the subject, I have found it 

 necessary to keep the following distinctions constantly in mind : 



(1) The selection that results in the transformation of species is not 

 the selection of one species to the exclusion of another. The breeding 

 of the horse to the exclusion of the ass modifies neither the one nor the 

 other. It is the exclusive generation of certain variations of a single 

 intergenerating group that gradually transforms the group. When, 

 therefore, we speak of selection as a cause of transformation, we refer 

 to the selection of the variations that are to interbreed and keep up the 

 race, to the exclusion of other variations. In order to maintain the 

 same distinction in the nomenclature of natural processes, what I call 

 " selection" is caused by the failure of certain forms of a species to 

 perpetuate their kind as contrasted with the success of other forms. 

 If the failure includes all the forms of a species, I call it the extinction 

 of that species and class it as a cause of transformation in the remain- 

 ing species only so far as it makes a change in their environment. 



(2) The exclusive generation of certain forms of an intergenerating 

 group does not necessarily result in transformation. Experiments in 

 artificial breeding show that if we select only the typical representa- 

 tives of a race the general character of the race is not changed, though 

 any tendency to fluctuating variation may be gradually diminished 

 and the stability of the type increased. When, however, one form of 

 deviation from the mean is constantly selected without a counterbal- 

 ancing selection of the opposite deviation, the transformation of the 



