Progression, Retrogression and Degression. 75 



often cannot 'tell, even from the best illustrations, to 

 which genus or family a given instance belongs. 



I shall therefore throughout this Part attempt to 

 describe the origin of horticultural varieties as exhaus- 

 tively as possible. In the present state of our knowledge 

 they form in my opinion the pattern of retrogressive and 

 degressive formation of species; just as the mutations 

 of OejiolJiera were the pattern of progressive changes. 

 Together they give us some idea of the main lines along 

 which specific differentiation takes place in nature, at the 

 present time as well as in the past. 



In conclusion : Progress on the main lines of descent 

 results from the production of new characters; but the 

 extraordinary variety of forms results from the occa- 

 sional disappearance of characters already existing, or 

 from the activation of latent ones (retrogression, de- 

 gression, atavism). 



