286 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 



and so on, for other sorts of combinations of other characters. 

 Any set of characters might vary independently of the rest. 



5. The hereditary variations which arose were of just such a 

 nature as to produce from a single strain the hereditarily different 

 strains that are found in nature. 4 



I judge that if the intermediate strains were killed, the two 

 most diverse strains found in nature might well be classed as 

 different species, although the question of what a - species is 

 must be left to the judgment or fancy of the individual. 



Such then were the results of my own studies as to the nature 

 of hereditary variations and how they appear. How do these 

 results compare with those found by other men? If we take a 

 general survey, we find the following main classes of cases: 



1. First, we have the mutations of Oenothera and its relatives: 

 large transformations occurring suddenly. Here is evidently 

 one of the most interesting fields of genetics, but I cannot feel, 

 in view of many extraordinary phenomena in this group, that 

 the bearing on the main problems of genetics is yet clear. 



2. Second, we have a large miscellaneous collection of muta- 

 tions observed in various classes of organisms: "bud variations," 

 dropping out of unit factors, and the like — all definite saltations, 

 but not genetically fully analyzed. 



3. In Drosophila as studied by Morgan and his associates, we 

 have the largest and most fully analyzed body of facts which 

 we possess with respect to changes in hereditary character in 

 any organism. The changes here are pictured as typical salta- 

 tions ; but of these I shall speak farther. 



4. In paleontology, as the results are presented in recent 

 papers by Osborn, 5 the evidence is for evolution by minute, 

 continuous variations which follow a single definite trend. 



5. Finally we have the work in biparental inheritance from 

 Castle and his associates: 6 this, as interpreted by Castle, gives 

 evidence for continuous variation, not following a single neces- 

 sary trend, but guided by external selection. 



4 The full account of this work is given in Jennings, 1916. (See Bibliography.) 



5 See Osborne, 1912, 1915, 1916. (See Bibliography.) 



6 See Castle, 1915 a, 1916, 1916 a, 1916 6, 1917; Castle and Phillips, 1914, 

 etc. (See Bibliography.) 



