

directly attributable to self-fertilization, but must be due to an 

 indirect effect. The distinguishing characters of these two rows 

 are permanent inheritable qualities and each therefore represents 

 what is known as an elementary species or biotype, as Johannsen 

 has appropriately named the elementary form-group. Self-fer- 

 tilization has simply isolated the two described forms by separating 

 them from their hybrid combinations with other elementary species. 



By rearing under different .conditions or by selecting in different 

 directions f it is possible to get a number of somewhat different 

 strains within the same biotype; but, if we do not distinguish 

 clearly between biotypes and these strains which differ only because 

 of the different treatment they have received, only confusion can 

 result. The difference between biotypes and the different strains 

 of a single biotype lies in the nature of the characters which they 

 possess as regards their heritability. Two biotypes will remain 

 distinct from each other without resort to selection as long as- they 

 are kept pure-bred and are grown under like conditions, provided 

 those conditions are sufficiently favorable generally to allow such 

 characters as each possesses to develop normally. Two strains 

 within the same biotype may be just as distinct from each other 

 as some biotypes are, but when they are grown under the same 

 conditions, constant selection will be required to keep them dis- 

 tinct, and if selection is' omitted the distinguishing characteristics 

 quickly disappear, usually within several generations. Such char- 

 acters as can only be retained in a pure-bred race by constant se- 

 lection or by culture under a particular set of external conditions 

 are called fluctuating characters. Now, the inheritance of the 

 fluctuations in any character follow a well-known law, usually known 

 as Galton's law, whose essential feature is the lagging of the average 

 value of single characters of the offspring behind the average of 

 the parents with respect to the same characters. In other words, 

 the average condition of children with respect to any fluctuating 

 character, stands between the average condition of the parents 

 and the average condition of the biotype to which they belong. 



It follows from this law that when a given degree of a fluctuating 

 condition is continuously selected under fairly constant cultural 

 conditions, the ideal which is followed in the selection marks the 

 theoretical limit of progress which will take place in the direction 

 of that ideal, and there will always be some lagging back of the aver- 

 age condition, which lagging becomes less and less, the longer the 

 selection is continued. To be specific, in the strain of maize with 

 which I started the most frequent number of rows of grains per ear 

 was 14. According to theory, if we are dealing here with a fluctuat- 

 ing character of a single biotype, we should never expect to be able 

 to exceed 20 rows on the average, by continuously selecting 20- 

 rowed ears for seed, and in like manner we could never hope to get 

 a strain whose average number of rows is less than 12, by contin- 

 uously selecting 12, provided no change in the conditions of the cul- 





