PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 887 



is able to do this, it invariably reduces or perhaps entirely eliminates 

 other species in the locality. Evolutionary changes result from 

 survivial of the fittest which has come about by accumulation of 

 favorable variations in each of successive generations. Those in- 

 dividuals or races which have not been as well adapted to conditions 

 of the habitat have become inconsequential or extinct. The appear- 

 ance of characters in an animal is a matter of chance as far as the 

 individual is concerned. Natural selection may act as an eliminating 

 agent and determine w^hether the character or trait shall survive after 

 it appears. 



Mutation Theory. — ^Hugo de Vries is the name most prominently 

 connected with the origin of this theory. He was a Dutch botanist and 

 in 1886 found some evening primrose plants (Oenothera lamarkiana) 

 which exhibited discontinuous variation or sudden appearance of 

 new characters. These sudden sharp variations came to be known as 

 mutations. There are two types of variations recognized: (1) con- 

 tinuous or fluctuating variation, such as height of individuals of a 

 species where they are expected to fall within a normal range thus 

 allowing a degree of variation among individuals of the species, and ; 

 (2) discontinuous or sport variation (mutation) where the variation 

 falls outside the normal range of variation and not connected with 

 it by intermediate changes. 



In the course of several generations of this primrose and involving 

 approximately 50,000 individuals, six different mutations were found. 

 The new characters which appeared, were quite different from those 

 of the typical species as w^ell as being inherited. Since this was the 

 case, de Vries concluded that he was observing the origin of new 

 species. He was sufficiently convinced of this to discount Darwin's 

 conception of the development of new species by the gradual accumu- 

 lation of continuous variations through natural selection. He pointed 

 out that mutations are due to changes occurring in the germ plasm 

 while the continuous variations, individually, are due to changes in 

 somatic cells. 



Following de Vries' work there has been much study of mutations 

 and numerous ones have been found in nature. Too, it has been 

 found that spontaneous mutations could be produced in Drosophila 

 (fruit fly) by x-ray radiation. It is thought that mutations come 

 as the result of physiological changes in the chromosomes or genes. 



