ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 131 



as a concentration of the mesogloea, due to the pressure of the collo- 

 sclere, or as a precipitation membrane formed at the surface of contact, 

 between the gelatinous collosclere on the one hand, and the gelatinous 

 mesogloea on the other, the former consisting of colloidal silica, and the 

 latter presumably of an albuminoid character, both probably with small 

 quantities of mineral salts. 



Protozoa. 



Heredity and Variation in Difflugia.* — H. S. Jennings has made 

 an intimate study of heredity, variation, and the results of selection in 

 the uniparental reproduction of Difflugia corona. This Rhizopod shows 

 a number of very definite characters that are congenital, not modified by 

 growth, and not affected by the environmental conditions during the 

 life of the individual ; these are therefore remarkably favourable for 

 studies of inheritance. These characters are : (1) the number of the 

 spines on the shell ; (2) the length of the spines ; (8) the diameter of 

 the shell ; (4) the depth of the shell ; (5) the number of teeth surround- 

 ing the mouth ; and (6) the diameter of the mouth. 



In a population found in nature the individuals differ among them- 

 selves in all these characters. In large populations an increase in any 

 one of the characters is accompanied on the average by an increase in 

 the others. When a population is allowed to propagate, the charac- 

 ters of the parents are inherited in a high degree by the progeny. 

 Co-efficients of correlation between parent and offspring rise even to 

 • 9 with respect to some of the characters. 



Populations consist of many hereditarily diverse strains. The 

 heritable characteristics of a given strain show a high degree of con- 

 stancy through many generations, though the individuals within the 

 strain may differ greatly in their personal cliaracters. The strains 

 show hereditary diversities with respect to the six characters enumerated 

 above, and with respect to the way in which these are combined. A 

 strain that shows one of the sets of hereditary characters in a higher 

 degree may show another one iti a low degree. The combinations 

 distinctive of particular strains cannot be accounted for as due merely to 

 the difference in some one underlying character, such as the size of the 

 body. 



When a single family is studied by itself (all descended by fission 

 from one original parent), it is found that a considerable degree of 

 correlation obtains between parent and offspring for most characters. 

 In regard to some characters (e.g. the number of teeth, often also the 

 diameter) the correlation of parent and offspring is very high (at times 

 • 5 or more). Sometimes this high con-elation is due to a mere steady 

 increase in size from generation to generation. But in most of the 

 families the correlation is due to the inheritance of parental diversities. 

 Selection for diversities within the single family, e.g. as to number 

 of teeth, was found to be effective. In regard to the size of the animal 

 -and the length of the spines, selection is much more rapid than in regard 



* Genetics, i. (1916) pp. 407-534 (19 figs.). 



K 2 



