CAUSES OF VARIATION 103 



are interbred. As Prof. Cossar Ewart says * : " Domestic animals 

 reproduce themselves with great uniformity if kept apart ; but the 

 moment one mixed up two different races, strains, or breeds, one did 

 something that was difficult to put in words, but the result was what 

 has been best described as an ' epidemic ' of variations." 



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

 amphimixis acts as a check on variability, 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 continuous 

 propagation of these qualities and these forms." Similarly Darwin 

 said : " When species are rendered highly variable by changed con- 

 ditions 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 chro- 

 mosomes 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 combinations in th& 



mature germ-cells is |- i, and the number of possible combinations 



(n \ a n* 

 -+lj = + w + !. 



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

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

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

 germ-cells is 5, and the number of theoretically different offspring is 

 25, i.e. on the assumption that the chromosomes are heterogeneous. 

 But according to the laws of chance certain combinations 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 



* Discussion on Heredity in Disease, Scottish Med. and Surg. Journal, vi 

 1900, p. 308. 



