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THE AMEBIC AN NATURALIST 



[Vol. XLVI 



definite characters— i. e., certain pure-line homozygotes 

 —are sufficiently constant in successive generations to 

 be expressed by a fixed standard, well and good. The 

 whole heredity shorthand is then simple. If such is not 

 the case, the character must still be described by some 

 fixed standard, but in that case recourse must be had to 

 complex mathematical expressions and not to a single 

 unit to describe the most constant somatic expressions. 

 Furthermore, if these mathematical expressions served 

 any practical purpose, it would be necessary to prove 

 that all somatic variability of homozygotes under uni- 

 form conditions (if there is any) may be expressed by 

 very few formulas. 



Such an attitude does not seem to be in harmony with 

 the progressive spirit of the times. I believe that we 

 may describe our results simply and accurately by hold- 

 ing that unit factors produce identical ontogenetic ex- 

 pressions under identical or similar conditions. If under 

 identical conditions the expression is different, then a 

 new standard, a new unit, must be assumed ; that is, fac- 

 tor A by any change becomes factor B. The results of 

 the pure-line investigations are the warrant for this in- 

 terpretation, for they are the investigations of suceess- 

 sive generations of somatic expressions with the least 

 attendant complication. From them one may assume 

 that a succession of individuals homozygous in all char- 

 acters and kept under identical conditions will be alike. 4 

 To be sure there are numerous changes in the expression 

 of characters when external and internal conditions are 

 not so uniform as the above, but I believe that these 

 changes can all be described adequately and simply by 

 ascribing them to modifying conditions both external 

 and internal. When external we recognize their usual 

 effect in what we called non-inherited fluctuations, when 

 internal we recognize their cause in other gametic fac- 

 tors inherited independently of the primary factor but 



4 Possibly even under these conditions rare variations that are exceptions 



