MUTATION; ORTHOGENESIS 963 



from one another and the resulting comhinations or elementary 

 species are likewise distinct and definite and without transitional 

 connecting forms. They are constant and transmit their characters 

 truly. The sudden appearance of these new forms is a process 

 which De Vries calls "mutation," thus using Waagen's term for a 

 process instead of a result, as originally proposed. The "elementary 

 species" as defined by De Vries is, in a measure, identical with the 

 mutation of Waagen, in that the variation is a slight and definite 

 one ; but in so far as De Vries believes in the stability and immuta- 

 bility of the elementary species, they do not correspond to the muta- 

 tion and sub-mutation (allometrons) as used by most palasontolo; 

 gists to-day. 



Orthogenesis and the Concept of Species. 



The doctrine of definite directed variation, or Orthogenesis, 

 which finds many adherents, especially among palaeontologists, has 

 led to a very logical conception of the method by which species 

 become dififerentiated. Though independently formulated with 

 more or less precision by many naturalists this doctrine was most 

 consistently and vigorously championed by the late Professor Theo- 

 dor Eimer of Tiibingen. Eimer's illustrations were chiefly drawn 

 from the color patterns of recent animals, especially lizards and 

 butterflies (9). He found that the color patterns of organisms may 

 be reduced to four types, which always appear in the individual 

 development in a definite succession, viz. : ( i ) Longitudinal stages, 

 (2) spots, (3) cross stripes, and (4) uniform coloration. Each 

 succeeding type is developed out of the preceding one and replaces 

 it to a greater or less extent. When in a large number of indi- 

 viduals all developing in the same direction (orthogenetically) a 

 complete cessation of development occurs in different groups at indi- 

 vidual stages, the individuals thereafter only increasing in size, but 

 not changing, a large number of distinct species will originate which 

 differ from one another to the extent by which one group continued 

 to develop beyond the other. If a number of characters develop, 

 each in a given direction, and at a given rate in a large group of 

 individuals, all starting from the same point, cessation of develop- 

 ment of different characters at different times will soon result in 

 the formation of a great number of species varying in one or 

 more characters. 



We may assume, by way of illustration, a case in which there 

 are three structural characters, which we may designate characters 

 (a), (b), and (c), in a given group of individuals, each changing 



