Castle: Pure Lines and Selecti 



ON 



95 



of the biologist, especially if some means 

 can be devised to test their validity by 

 experiment. 



Since the supposed "factors" of in- 

 heritance are invisible, we can not hope 

 to deal with them directly by experi- 

 ment, but only indirectly. Our method 

 obviously should be to eliminate all en- 

 vironmental factors so far as possible 

 and also all factors of inheritance except 

 one. If then this one being present 

 gives a uniform result and being absent 

 gives a result also uniform but different, 

 we can conclude the factor constant. 

 But it is very difficult to apply this 

 method to specific cases, since when 

 variation is observed it is always pos- 

 sible to suppose that all factors but one 

 have not yet been eliminated. 



From existing experimental work 

 what evidence have we for the idea of 

 factorial constancy? Perhaps Johann- 

 sen' has contributed more than any 

 other one person toward popularizing 

 the idea of factorial constancy. Select- 

 ing size variations in bean seeds, he 

 was sometimes successful in modifying 

 the racial mean, sometimes unsuccess- 

 ful. Whenever he was successful, he 

 attributes the success to variation in 

 genetic factors; whenever unsuccessful 

 he assumes that no variation in genetic 

 factors occurred, i.e., that a pure line 

 had been established, which could not 

 subsequently vary unless hybridization 

 occurred out of the race or a mutation 

 occun^ed within it. It would be of 

 much interest to know whether Johann- 

 sen's pure lines were as pure for all 

 other characters as for size of seed and 

 equally unresponsive to selection in all 

 particulars of leaf and stem. For the 

 bearing of his observations on seed size 

 upon the question of the constancy of 

 Mendelizing characters is not very 

 obvious, since seed size does not Men- 

 delize. 



NO CLEAR-CUT DEMONSTRATION. 



Although extensive observations upon 

 the subject of size inheritance in both 

 animals and plants have been made, 

 they have resulted in the demonstra- 

 tion as vet of no single clear-cut Men- 



dcHzing unit character (or factor either). 

 Dwarf plants, known to Mendelize when 

 crossed with tall ones, form only an 

 apparent exception; in reality they differ 

 in habit rather than in size from tall 

 ones. 



The results of all observers, as regards 

 the inheritance of ordinary differences 

 in size, are closely in accord. When two 

 races differing in size are crossed, the 

 immediate offspring are intermediate in 

 size. The next generation of offspring 

 is likewise intermediate but more vari- 

 able as a rule, and it has been found 

 possible in some cases to select from 

 them forms as extreme in size as the 

 original parents. To interpret such 

 cases as Mendelian requires the assump- 

 tion that no single unit or factor is con- 

 cerned in the size difference, but manv 

 wholly independent units. For a single 

 Mendelizing unit would produce a 

 wholly different result. But suppose 

 we allow the assimiption that many in- 

 dependent Mendelizing units or factors 

 are concerned in the inheritance of size. 

 The pure line hypothesis is not bene- 

 fited by this assumption, unless we sup- 

 pose further that these hypothetical 

 factors do not vary. But this is an 

 assumption wholly without warrant. 

 For in all cases studied critically with 

 reference to the constancy of characters 

 demonstrably Mendelian, the charac- 

 ters have been found to be inconstant 

 and subject to modification by selection. 

 What ground is there, then, for suppos- 

 ing that in a case where no factors are 

 demonstrable, such factors are invari- 

 ablef This is like supposing that the 

 moon is made of cheese and that further 

 this cheese is green. The speculation 

 is harmless, if one chooses to amuse 

 himself that way, but it can scarcely be 

 called a valid scientific conclusion. 



Aside from the size selection experi- 

 ments of Johannsen, which were made 

 on a self -fertilizing plant, the experi- 

 ments of Jennings" are most often cited 

 in support of the view that selection 

 within the pure line is without effect. 

 In soine respects the material used by 

 Jennings was even more favorable than 

 that of Johannsen. It consisted of a 



'Elemente der exakten Erblichkeitslehre, Jena, 1909. 

 ••Am. Naturalist, Vol. 43, No. 510, June 1909. 



