324 EEV. J. T. GULICK ON 



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 variation?. 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 trans- 

 formation in the remaining species only so far as it makes a change 

 in their environment. 



(2) The exclusive generation of certain forms of an inter- 

 generating group does not necessarily result in transformation. 

 Experiments in artificial breeding show that if we select only the 

 typical representatives 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 in- 

 creased. When, however, one form of deviation from the mean 

 is constantly selected without a counterbalancing selection of the 

 opposite deviation, the transformation of the race is always the 

 result. In other words. Balanced Selection produces Stability of 

 Type, and Unbalanced Selection produces Transformation of 

 Type. 



In the light of this twofold law we see how there may be 

 stringent Natural Selection without transforming effect. It has 

 sometimes been maintained that the transformation of species 

 through the Natural Selection of favoured races is a necessary 

 process which must be operating in nearly every species ; for in 

 nearly every species there is a constant sti'uggle between the 

 different forms of variation ; and as it never happens that all the 

 forms are equally successful, the process of Natural Selection is 

 always bearing in full force upon the species. If it could be 

 shown that Natural Selection, wherever it exists, must neces- 

 sarily produce transformation, it would be impossible to resist 

 the conclusion that nearly every species is undergoing transfor- 

 mation through this cause. But it is Unbalanced, and never 

 Balanced, Selection that produces transformation. We also see 

 that heredity tends to make the most successful form the average 

 form, and thus to convert Unbalanced into Balanced Selection. 

 Trom this it follows that in order that Selection- should pro- 



