Februakt 3, 1922] 



SCIENCE 



113 



tlie pistil. Perhaps the best examples of wind 

 pollinated types very freely open to outside 

 pollination are the numerous races and forms 

 that make up the collective species Zea Mays. 

 The studies of East and Jones, Emerson, Shull, 

 G. N. Collins and others, extending over many 

 years, show conclusively that corn is usually a 

 hybrid composite with so many characters rep- 

 resented by genes in single doses that purifica- 

 tion of material by selective inbreeding is a 

 matter of much time and patience. There 

 could hardly be a greater contrast in genetical 

 behavior than that between lines of wheat 

 which, because they rarely outcross, breed very 

 true, and races of corn that can only be kept 

 reasonably true by constant watchfulness, 

 practiced selection, and a never-ending elim- 

 ination of products departing from the types. 

 Self-sterility and the production of weakened 

 generations following inbreeding, as factors 

 leading to the establishment of impure species, 

 have not as yet received recognition propor- 

 tionate to their importance. Genetical studies 

 seem likely to show that there are large groups 

 of bisexual plants the individuals of which are 

 either infertile when selfed or produce 

 progenies in successive generations distinctly 

 inferior in vigor to the wild types. In such 

 material the species represented in Nature 

 must be very largely, if not wholly, made up 

 of individuals cross-bred and genetically im- 

 pure. It is significant that these conditions 

 should have been found in that most successful 

 assemblage, the Compositae, frequently cited as 

 the climax group of plant evolution. The 

 recent studies of Stout on chicory have shown 

 the extensive presence of self-sterility, and 

 that the wild populations must consist chiefly 

 of outbred and probably heterozygous indi- 

 viduals. Investigations of J. L. Collins on 

 Crepis indicate that species of this genus are 

 impure since progeny from selfed lines show 

 marked deterioration from the wild stock as 

 segregation proceeds and forms approaching 

 purity of germinal constitution are isolated. 

 Crepis seems likely to prove an assemblage of 

 impure species similar to that assemblage of 

 impure races called Zea Mays, and will prob- 

 ably show the same parallelism of behavior in 

 reduced vigor and the production of abnormal 



types as inbred lines are separated from the 

 wild population. The interpretation for 

 Crepis is likely to be that of East and Jones 

 for maize, namely, that inbreeding gives dele- 

 terious results through the segregation of 

 types with fewer genes for characters associ- 

 ated with physiological vigor of expression. 

 These studies are tending towards conclusions 

 well established for many cultivated fruits, as 

 apples, pears, plums, cherries, etc., where self- 

 sterility among the varieties proves to be the 

 rule and cross-pollination is necessary for 

 sexual reproduction through impure lines. It 

 is hardly possible that chicories and species of 

 Crepis are outstanding exceptions to condi- 

 tions in the Compositaa and we may safely 

 predict that studies in this immense assem- 

 blage will reveal wide-spread the presence of 

 impure species. Self-sterile lines among the 

 grasses have also been reported, e. g., Lolium 

 perrene. 



There is another type of impure species not 

 represented in the animal kingdom but common 

 in certain groups of plants and therefore of 

 particular interest to the botanist. This is the 

 hybrid which perpetuates itself by vegetative 

 means and thus establishes populations in the 

 wild when its characters are favorable to sur- 

 vival under the scrutiny of natural selection. 

 The well known principle of hybrid vigor, or 

 heterosis, may in itself be expected to give to 

 such hybrids marked advantage. These impure 

 species hold true to their characters through 

 asexual reproduction although by their seed 

 they may produce a large variety of segre- 

 gates. This principle of the maintainance of a 

 hybrid by vegetative reproduction is applied in 

 agriculture when selected lines of potatoes are 

 propagated from slices of the tubers and 

 strawberries from plants developed by the 

 runners, and in fruit culture by the grafting 

 of choice hybrid varieties. 



There have been two notable systematic 

 studies in America on groups of wild species 

 in which hybrids are found well established as 

 impure species. Brainerd's investigations on 

 the violets and blackberries show the possibili- 

 ties of critical studies on the status of species, 

 making use of the experimental garden and 

 basing results on genetical analyses. Favored 



