94 



Thk joiRXAL OF Heredity 



made to var}' 1:)\- the cn\-ironmcnl. 

 But as the effects of environment are 

 by hypothesis not inherited, all would 

 breed alike regardless of their appear- 

 ance, hence it would be useless to make 

 any choice among them. 



The pure line theory, as outlined 

 above, like the theorems of the geometer, 

 rests upon the \'alidity of certain postu- 

 lates, any one of which being dis])roved, 

 the whole argument fails. 



The pure Hne postulates are: (1) The 

 effects of environment are not inherited: 

 (2) inherited characteristics do not vary. 

 The first of these postulates is a bone of 

 unending contention among biologists, 

 and the last word has not been said upon 

 it yet. Practical breeders believe that 

 good care and feeding are not wasted 

 upon their stock, that not only the indi- 

 viduals so treated are benefited but 

 subsequent generations also. It must 

 be admitted, however, that the evidence 

 is not satisfactory that subsequent gen- 

 erations are given a better endowment 

 by better treatment of the jjresent 

 generation. What is inherited is capac- 

 ity to utilize o]j])ortunitics of the en- 

 \-ir(mmcnt. It is doubtful whether 

 utilization of the environment by one 

 generation increases the capacity of the 

 next generation so to utilize it, though 

 we all ojjtimistically hope for this out- 

 come. Plant breeders have shown that 

 plumjj, well-nourished seed jjroduces 

 more vigorous i)lants and larger harvests 

 than shrunken ill-nourished seed, a 

 seeming inheritance of environmental 

 effects. But in reality the seed is only 

 part embryo; part of it is food stuff, 

 environment of the i^rcvious generation 

 carried over bodily to form the early 

 en\'ironment of the new generation. So 

 that when we select well nourished seed 

 we select, not only a bundle of inherited 

 plant qualities, but also a good initial 

 environment for the ])lant. ( )n the 

 whole the first postulate of the jjure line 

 theory stands, if not proved, at least 

 not disproved. 



The second postulate of the i)uri- line 

 theory is a much shakier one, but I must 

 hasten to qualify the form in which it 

 has been stated iX'forc it is disowned 1)\- 



all sujJixjrters of the theory. The 

 Hagcdooms, who ha\-e championed the 

 theory in The American Breeders' A/ag- 

 azine (Vol. 4, No. 3), are careful to say 

 that inherited "factors, not characters", 

 are constant. What they mean, I take 

 it, is this. An animal has some visible 

 character, such as black fur. The black 

 color of this fur may, howe^•er, result 

 from several indeiicndent physiological 

 processes, or agencies, no one of which 

 by itself jjroduces a visible effect. These 

 agencies, known or unknown, arc factors 

 in producing a black coat. Some of 

 them may be inherited, others environ- 

 mental. Although the blackness of the 

 coat may vary (owing to variation in 

 environmental factors) its tdtimate in- 

 herited factors do not vary. 



EXPERIENCE WITH MEXDELIS.M. 



What evidence is there for the idea 

 that the ultimate factors of inheritance 

 do not vary? First let us inquire how 

 the idea originated. Ten years ago, 

 when the rediscovered Mendel's law 

 was new, it was supposed that characters 

 which confonned with Mendel's law did 

 not vary. They l^ehaved as units in 

 heredity; how could the>' vary; how 

 could units vary!" The gametes were 

 "pure". Each one either contained or 

 did not contain a particular Mendelian 

 unit. Units could not be S])Ht or modi- 

 fied. This idea found its fullest devel- 

 opment in Punnett's Mendelism.^ Later 

 investigations have shown beyond ques- 

 tion that Mendelian characters do vary. 

 Practically everyone has now abandoned 

 the idea of "gametic purity' \ but the 

 idea of purity has been shifted from the 

 characters which can be seen to vary, 

 to factors which may be imagined to be 

 invariable, though they can not be seen. 



We might at this ])oint with ])roi3riety 

 end the discussion concluding that 

 factors which are imaginary, like the 

 circles of the mathematician, are of ne- 

 cessity perfect and constant. This is 

 l^ractically the burden of East's arg- 

 ument in The American Naturalist'^. 

 But as the world has benefited l^y the 

 speculations of the geometers, so, let us 

 h()])e, it may profit by the S])eculations 



'The Macmillan (%.., N. Y., 1<)11. 

 •Vol. 46. No. .S51. Nov. I';i2. 



