30 The Mechanism of Evolution in Leptinotaksa 



variations of the same order may appear. Sudden variation in single characters 

 may also give rise to minor groups, or may change in nature the aspect of the 

 whole lineal group. 



In this material I have recognized and determined the extent to which any of 

 the following grades or minor groups are present in any specific form which is 

 to be used for experimental investigation before it is utilized in experimental 

 study. This analysis of the composition of the materials is absolutely necessary 

 and it is only by careful analysis of the composition that is is possible to avoid 

 frequent error and misinterpretation in the progress of investigation. In 

 practice I have found the following scheme of description and analysis to be 

 effective : 



Primary Grade — Spkcies: 



A. Gametic in character, or due to actual factorial composition of the organisms. 



Changes therein by loss, addition, recombination or transmutation. 

 1. Species: A group of organisms separable from all other groups by a 

 constant array of qualities, and genetically perpetuated in nature 

 throughout their range of habitats. 

 Secondary Grade: 



a. HaMtudinal varieties: A secondary and geographically localized 

 portion of the primary group which is distinguished from the 

 primary group by the possession of difference in, or accentuation 

 of the characters of the primary group in the geographic area 

 inhabited by the differentiated portion of the species, and geneti- 

 cally constant in nature. 

 6. Recurrent ''mutations'' : Periodic, or recurring discontinuous varia- 

 tions in the group, which appear as sudden deviations, independent, 

 genetically perpetuated races. 



c. Saltations (sports): Occasional variation of greater or less magni- 



tude appearing without visible cause and often becoming the basis 

 of races of permanency and of permanent modifications in the 

 specific group. Usually involve sudden change in several associated 

 characteristics. Rare and irregular in occurrence. 

 Tertiary Grade: 



d. Biotypes: Lines of descent in which one or more attributes or 



qualities are accentuated or suppressed and capable of isolation 

 and propagation by breeding. Such biotype groups present inter- 

 grading variations with the remainder of the population and the 

 " purification " results in reducing the fluctuation of the type. 

 Equivalent to the Biotype of Johannsen, Elementary Species of 

 DeVries, and many of the " varieties " of domesticated plants and 

 animals. 



B. Somatic in character: Not due to factorial change but to developmental 



alteration of the somatic expression of gametic factors. 



a. HaMtudinal somatotypes: Somatic alterations recurrently produced in 



any form in successive generations by influences present in the 

 habitat without producing gametic change. Not inheritable and 

 always lost on change of habitat. Variable. Character only 

 decided by breeding in changed environment. 



b. Environmental (aberrations) : Irregular discontinuous somatic changes 



produced by sudden irregularities in the medium or other excessive 

 conditions. Not inheritable. Character of change only proven 

 after breeding tests. 



This arrangement of the divisions is applicable to species in a state of nature, 

 where genetic continuity and stability are demanded. Often there are found 

 year after year, associated with some species, certain types, and these are 

 described as varieties or even as species — e. g., L. melanothorax Stal — a pro- 

 cedure that is entirely justifiable on the basis of the information ordinarily 

 available ; but not infrequently I have found that these types are not genetically 

 continuous, but are produced anew as sports recurring in each generation, or 



