The Theory of Evolution 227 



generations would soon lead to the production of a large number 

 of individuals of the new type. The possibility of the forma- 

 tion of a new species in this way is clearly shown. The pro- 

 portion of individuals belonging to the new and to the old type 

 varies greatly in different forms, and there appears to be a curi- 

 ous relation between the type produced and the sex of the indi- 

 vidual, but with this question we are not here concerned. A 

 single example will suffice to illustrate the character of the result. 

 The moth Aglia tau produces an aberrant form, A. lugens. A 

 cross between the two types produced 14 males and 28 females 

 of A. tau, and 31 males and 13 females of A. lugens. Two indi- 

 viduals (Fj) of the type A. lugens belonging to the second genera- 

 tion (F 2 ) were mated. 1 They produced 3 males and 8 females 

 of A. tau, and 49 males and 42 females of A. lugens. The results 

 do not appear to conform to the Mendelian law, but it is not 

 improbable that the aberration was not a pure form but a domi- 

 nant-recessive. However this may be, the appearance of both 

 types and the absence of intermediate grades is of the highest 

 interest. 



Standfuss's general conclusion is given in the following state- 

 ment: A union between the parent species and an aberration 

 that has arisen suddenly and discontinuously ("sprungweise") 

 a variety in the technical sense produces in many cases not 

 any intermediate forms, but the parental species type and that of 

 the aberration. On the other hand, a union between the parent 

 species and an individual of a local race showing a series of inter- 

 mediate gradations produces a series of intermediate forms. 2 



If the new character is transmitted to all of its hybrid offspring 

 and to their descendants, it will become sooner or later trans- 

 ferred to all of the individuals that are met with, and in time may 

 become a part of the common inheritance. 



It remains only to consider another point one on which 

 de Vries has recently laid some emphasis. He finds in the even- 

 ing primrose that the new progressive mutations are to some 



1 Of another combination, however. 



2 This is the case apparently in the crosses described by Tutt. 



