CAUSES OF VARIATION 103 



On the other hand, Hatschek and others have pointed out that 

 amphimixis acts as a check on variabiHty, obviating heterogeneous 

 idiosyncrasies. This was suggested even by Lamarck : "In repro- 

 ductive unions the crossings between the individuals which have 

 different qualities or forms are necessarily opposed to the con- 

 tinuous propagation of these qualities and these forms." Similarly 

 Darwin said : " When species are rendered highly variable by 

 changed conditions of life, the free intercrossing of the varying 

 individuals tends to keep each form fitted for its proper place in 

 nature." 



Combinations of Chromosomes. — Prof. H. E. Ziegler has given 

 much attention to the number of possible combinations of parental 

 chromosomes in the offspring, supposing the distribution to be 

 fortuitous. If the normal number of chromosomes in a species is n, 



the number of tetrad groups is - , the number of possible combina- 

 tions in the mature germ-cells is - + i, and the number of possible 



combinations in the fertilised egg-cell is ('- + i V = - + n + 1. 



If the normal number of chromosomes be 8 (as in the fluke often 

 found parasitic in frogs, Polystomum iniegerrimum), the number of 

 tetrad groups is 4, the number of possible combinations in the 

 mature germ-cells is 9, and the number of theoretically different 

 oiffpring is 25, i.e. on the assumption that the chromosomes are 

 heterogeneous. But according to the laws of chance certain com- 

 binations are much more frequent than others ; the larger the 

 number of tetrad groups the more frequent is the occurrence of an 

 approximately equal number of paternal and maternal chromosomes 

 in the germ-cell. 



Sutton puts the matter as follows. An individual receives from 

 his father 4 chromosomes. A, B, C, D, and from his mother (an equal 

 number) a, b, c, d. The immature germ-cell has A, B, C, D ; a, b, c, d. 

 These group themselves in four tetrads, each composed of two 

 double chromosomes, two maternal and two paternal, Aa, Bb, Cc, 

 Dd. The mature germ-cell receives one chromosome from each 

 tetrad, and there are 16 possible combinations — viz. a, B, C, D ; 

 A,b, C, D ; A, B, c, D ; A, B, C, d ; a, b, C, D ; a, B, c, D ; a, B, C, d ; 

 a, b, c, d ; and eight others which may be got by replacing small 

 letters by capital letters and vice versa. The number of possibly 

 different offspring would be 16^. 



