722 THE AMERICAN NATURALIST [Vol. L 



of different factors. It has been generally recognized 

 that this is a possible conception. The following authors 

 have considered this point : Castle and Phillips, :14, Mul- 

 ler, :14, Hagedoorn, :14, MacDowell, :15. 



2. The hybrid ancestry of the original parents affords 

 a source for a large amount of heterozygosis. It is the 

 reduction of this heterozygosis that selection is supposed 

 to accomplish, in separating the two races, plus and 

 minus. 



3. Such a reduction of heterozygosity would be hin- 

 dered by the large number of matings made between rats 

 less closely related than brother and sister. This point 

 has been discussed by Muller. 



4. By breaking the correlation between the soma and 

 the germ plasm, environment has probably played a large 

 part in hindering the reduction of heterozygosity. Ap- 

 parently it has been assumed that there is a close rela- 

 tionship between the germ plasm and the soma, that the 

 smooth curve of the averages, in successive generations 

 proves that the germinal variations, to which the rise in 

 the curve is due, are small and constantly occurring. 

 But, since the role of environment is not known, the 

 gradual advance in the averages can not prove anything 

 as to the size of the germinal variations. The presence 

 of regression makes it clear that environmental, or extra- 

 germinal, influences are active in producing variability 

 in the hooded pattern. Eegression is really a measure 

 of the degree of independence of the soma and germ 

 plasm. Regression expresses the inverse relationship 

 between the actually tested breeding possibilities and the 

 appearance of the parents. There can be no question as 

 to the activity of environmental influences ; of their power 

 and nature nothing seems to be known. The immediate 

 environment of the undifferentiated blastomeres is prob- 

 ably as important a factor in the final appearance of a 

 character as the germ plasm itself. The factors in the 

 germ plasm are like chemicals that will react in a definite 

 way in connection with certain other chemicals; when 



