534 THE AMEBIC AN NATURALIST [Vol. LI 



crosses in pigeons, Kiddle, confirming AVhitman's dis- 

 covery, finds that when an enforced series of eggs are 

 laid, their chemical composition is changed and that they 

 produce at certain times a preponderance of males. Since 

 the female here is the heterozygotic sex (ZW) the results 

 are such as would follow a direct influence on the sex 

 chromosomes when the polar body is eliminated. Infor- 

 mation concerning sex-linked inheritance in these forced 

 offspring should settle the question. 



To sum up, it may be said that ''plastid" inheritance 

 is at present the only known method of transmission of 

 factors that does not come under Mendel's laws. The 

 three principal kinds of Mendelian inheritance known at 

 present fall into the following groups : 



1. Autosomal inheritance, where transmission is equally 



to both sexes, or to all individuals of hermaphroditic 

 species. 



2. Sex-linked inheritance, (a) where the distribution of 



characters coincides with the distribution of the X 

 chromosomes in the Drosophila type, and of the Z 

 chromosomes in the Abraxas type ; and (b) where the 

 distribution of characters coincides with the distribu- 

 tion of the Y chromosome (as illustrated by the fer- 

 tility of the male of Drosophila that depends on the 

 presence of the Y chromosome) or of the W chromo- 

 some in moths. 



3. Inheritance due to unusual distributions of chromo- 



somes, as seen (a) in doubling of their number (tetra- 

 ploidy) ; (b) in non-disjunction, as in the 15-chromo- 

 some tjipe of (Enothera and the XX Y type of female 

 in Drosophila; {c) in irregularities of synapsis as 

 seen in species hybrids such as Pygmra. This group 

 (3) is at present only provisional and will no doubt 

 be broken up at some future time into its different 

 parts. 



The case of maternal inheritance, spoken of above (other 

 than Y or W linked or plastid inheritance), has been 



