Apeil 3, 1914] 



SCIENCE 



481 



and animal breeding, but also for society 

 in general. 



II 



The general results of the investigations 

 bearing upon the evolutionary control of 

 organisms may be grouped around the prin- 

 ciples of Mendelism, the mutation theory, 

 and pure line breeding. 



The rediscovery in 1900 of the funda- 

 mental laws governing hybridization so 

 brilliantly established by Mendel in 1865, 

 but unfortunately concealed in the obscure 

 publications of the Natural History Society 

 of Brunn, opened an extraordinary field 

 for experimental work. This has already 

 developed to vast proportions in connec- 

 tion with both the results obtained and the 

 speculations involved, while the end is not 

 in sight. 



The investigations of Mendel, now so 

 familiar to all biologists, and which may be 

 mentioned somewhat in detail here because 

 of their bearing on mutation, consisted 

 primarily in the crossing of tall and dwarf 

 peas, with the result that the first filial (Fi) 

 or hybrid generation consisted entirely of 

 tall plants. When, however, seeds from 

 these plants were sown the ratio of tall to 

 dwarf plants became 3 to 1 in the second 

 (Fj) hybrid generation, a result explained 

 by the theory of dominant and recessive 

 characters on the basis that there are cer- 

 tain determiners of unit characters in the 

 germplasm which dominate over others 

 during the development of the somatoplasm 

 or body of the individual in the higher 

 forms of life. More recently the presence 

 and absence theory has been applied in 

 interpreting the results. In a manner simi- 

 lar to the preceding when smooth yellow 

 peas were crossed with wrinkled green peas 

 the first hybrid generation consisted of 

 smooth yellow forms inasmuch as the char- 

 acter smooth and the character yellow were 

 dominant over the character wrinkled and 



the character green, and the crosses were 

 known as dihybrids, inasmuch as they dif- 

 fered in respect to two characters. In the 

 second hybrid generation the resultant 

 ratio was 15 to 1 pure recessive, i. e., 

 wrinkled green, although the fifteen con- 

 sisted of smooth yellow, smooth green, and 

 wrinkled yellow in the proportion of 9 : 3 : 3. 

 In the same way trihybrids have the ratio 

 63 to 1 pure recessive while any polyhybrid 

 differing in "n" characters which mendel- 

 ize in the usual manner will give an ex- 

 pected ratio of 4" — 1 to 1 pure recessive, 

 which will become apparent only through 

 the breeding of large numbers of indi- 

 viduals. 



While the preceding summary represents 

 the normal results in connection with the 

 segregation of unit characters, studies of 

 the past few years have demonstrated that 

 many interesting relationships may occur 

 between the factors governing the produc- 

 tion of characters. For example, it has 

 been found that two or more determiners 

 are often present either of which will pro- 

 duce the given character as Nillson-Ehle 

 demonstrated in hybrids of brown and 

 white chaffed wheat, while on the other 

 hand two or more determiners acting to- 

 gether may be necessary to bring about an 

 effect. Such a condition exists, as Bateson 

 in 1910 showed, in certain white-flowered 

 sweet peas which when crossed produce 

 purple flowers in the first hybrid genera- 

 tion. The results which have led to the 

 theory of coupling and of repulsion, par- 

 ticularly the latter, where the expectancy 

 of a pure recessive may be one among many 

 thousands, go far toward suggesting a pos- 

 sible explanation of many so-called muta- 

 tions on the basis of ancestral individuals 

 heterozygous for one or more characters. 



Do the Mendelian principles assist us, 

 however, in attaining the goal which we are 

 seeking, namely the building up of an ideal 



