THE PROBLEMS OF HEREDITY AND THEIR SOLUTION. 575 



But there are additjonal and even more sij>'nitieant doduetions from 

 the facts. We have seen that the o-amctes are difi'erentiated in re.spect 

 of pui'e eharacters. Of these pure characters there may conceivabl}^ 

 })e any numl)er associated together in one organism. In the pea Men- 

 del detei'ted at least seven — not all seen by him eoml)ined in the same 

 plant; but there is every likelihood that they are all capable of being 

 thus combined. 



Eacli such character, which is capal)le of being dissociated or replaced 

 by its contrary-, nuist henceforth ))e conceived of as a distinct unit 

 character; and as we know that the several unit characters are of such 

 a nature that any one of them is capable of inch^pendentl}' displacing 

 or being displaced ))V one or more alternati\e characters taken singly, 

 we may recognize this fact b}' naming such unit characters allelomorphs. 

 So far wo know very little of an}' allelomorphs existing otherwise 

 than as pairs of contraries, but this is probably merely due to experi- 

 mental limitations and the rudimentary state of our knowledge. 



In one case (coml)s of fowls) we know three characters — pea comb, 

 rose comb, and single comb, of which pea and single, or rose and sin- 

 gle, behave toward each other as a pair of allelomorphs, but of the 

 behavior of pea and rose toward each other we know as 3^et nothing. 



We have no reason as yet for affirming that any phenomenon prop- 

 erh' described as displacement of one allelomorph l)y another occurs, 

 though the metaphor may be a useful one. In all cases where domi- 

 nance has been perceived wt^ can affirm that the members of the 

 allelomorphic pair stand to each other in a relation the nature of which 

 we are as yet wholly unable to apprehend or illustrate. 



To the new conceptions already enumerated we may therefore add: 



(4) Unit characters, of which some, when once arisen l)y variation, 

 are alternative to each other in the constitution of the gametes, 

 according to a dehnite system. 



From the relatious subsisting between these characters it follows 

 that as each zygotic union of allelomorphs is resolved on the formation 

 of the gametes, no zygote can give rise to gametes collectively repre- 

 senting more thtui two chai-acters allelomorphic to each other, apart 

 from new variation. 



From the fact of the existence of the interchangeable chai'acters we 

 must, for purposes of treatment and to complete the ])os,sibilities, nec- 

 essarily form tlie conception of an irresoluble base, though whether 

 such a conception has any objective reality we have no means as yet of 

 determining. 



We have now seen that when [\\o varieties ^1 and // are crossed 

 together, the lieterozygolx^ A/) [)i'oduces gametes bcai'iiig the pure A 

 character and the pure 7/ characbM'. In such a case we speak of such 

 characters as simple allelomor})hs. In many cases, however, a more 

 complex j)henomen()n happens. 'Hw. character brought in on fertili- 

 zation b}' one or other })areiit may be of such a natui'e that when the 



