62 HELEN DEAN KING 



In full accord with these findings for mice is the series of 

 coefficients for litter size in captive gray rats, as given in 

 table 10 of the present paper. Little ( '33) states, "on a priori 

 grounds there is no reason why the germ cells, as an organiza- 

 tion, should not also show a durational phase in its various 

 activities." In this connection Bridges ('29) has given data 

 indicating that in Drosophila crossing-over varies with the 

 age of the mother, the greatest variation being shown by very 

 young and by very old females. Haldane and Crew ('25) 

 found that in poultry linkage between two dominant sex- 

 linked factors in males decreased steadily and had practically 

 disappeared by the third year. This finding, they say, "is of 

 interest as pointing to pre-senile changes in the behavior of 

 the dividing nucleus, and as being the clearest case so far 

 recorded in vertebrates of a change with age of the 'germ- 

 plasm' of an individual." 



That very young and very old germ cells may function more 

 variably than do those of the middle age period seems further 

 substantiated by the fact that in man some types of defects, 

 which seemingly have an hereditary basis, are found more 

 commonly among children of young mothers, while other 

 types appear more frequently among the late-born. Pearson 's 

 ( '14) analysis of a large series of data relative to the relation 

 of the order of birth to various defects showed that congenital 

 cataract, albinism, mental defects, tuberculosis and criminal- 

 ity have a particular tendency to occur among the first or 

 second children. On the other hand, Mongolism shows a dis- 

 proportionate frequency among the late-born, particularly in 

 large families, as many investigators besides Pearson have 

 shown (Langdon-Down, '06; Davenport and Allen, '25; Van 

 der Scheer, '27; Penrose, '32, '33; Murphy, '36, etc.). 



In the further development of his thesis Little ('33) also 

 states : 



It might well develop that tendency to mutation, which in 

 itself is an indication of variability, would be greater in very 

 young and in very old germ cells. If this were true, domesti- 

 cation, in which very young and very old animals, protected 



