Each organism, according to DeVries, consists of thou- 

 sands of elementary entities or units which combine to 

 give it its form and functions. Similarity of species con- 

 sists in the similarity of these elementary unit characters, 

 while divergent species contain one or more different units. 

 A unit may express itself in different organs, so the principle 

 of units may help to explain the large number of correla- 

 tions often discernible in plants. "The correlated external 

 marks may be assumed to be the expressions of the same 

 internal characters." 



In an organism a number of characters may be distin- 

 guished capable of varying independently and of being iso- 

 lated or followed separately in breeding experiments, each 

 depending on a special transmissible germinal factor or 

 factors. These characters are known as litiit characters. 



The idea of unit characters lies at the base of the mod- 

 ern theory of hybridism — Mendelism. "All the modes of 

 expression of one unit must steadily keep together whenever 

 the entire group of characters are thrown into one another 

 in crossing." 



Unit characters cannot be split or broken up, and 

 when a unit is added or subtracted from a type a mutation 

 is produced. 



It sometimes happens that a character will disappear 

 entirely in the course of time. Such a form is termed by 

 DeVries a retrograde variety When an entirely new char- 

 acter appears it is termed a Progressive Mutation. DeVries 

 considers this kind to have been the chief factor in evolu- 

 tion. 



When a character appears that has been latent it is 

 said to be Degressive Mutation. 



"Varieties differ from elementary species in that thev 

 do not possess anything really new. They originate for the 

 greater part in a negative way by the apparent loss of some 

 quality; and rarely in a positive manner by acquiring a char- 

 acter already seen in allied species" (DeVries, Species and 

 Varieties.) 



(a) — Correlation of Characters 



It is frequently observed that a particular color in a 

 flower IS associated with a particular taste or color in the 

 fruit or seed, or a particular color with a particular form 



Judges and students of live stock have long been famil- 

 iar with the fact that certain characters always go together 

 as if they were produced by the same cause and were ex- 



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