tance as well as for maternal inheritance, and it shows why 

 they may be of the same or equivalent degree. When, 

 now, the egg divides, at the first and later cleavages, the 

 chromatin masses or chromosomes contained in the double 

 nucleus are split lengthwise and the twin portions separate 

 to go into the nuclei of the daughter cells. As the same 

 process seems to hold for all the later divisions of the 

 cleavage-cells whose products are destined to be the vari- 

 ous tissue elements of the adult body, it follows that all 

 tissue-cells would contain chromatin determinants derived 

 equally from the male and female parents. As of course 

 only the germ-cells of an adult organism pass on to form 

 later generations, and as their content of chromatin is 

 derived not from the sister-organs of the body but from 

 the original fertilized egg, there is a direct stream of the 

 germ-plasm which flows continuously from germ-cell to 

 germ-cell through succeeding generations. This stream, 

 be it noted, does not flow circuitously from egg to adult 

 and then to new germ-cells, but it is direct and continuous, 

 and apparently it cannot pick up any of the body-changes 

 of an acquired nature ; indeed, it is doubtful whether such 

 changes can reach the germ-cells at all, for the path is not 

 traversed in that retrograde direction. 



It must be clear, I am sure, that this theory supplements 

 natural selection, as it describes the physical basis of inher- 

 itance, it demonstrates the efficiency of congenital or 

 germ-plasmal factors of variation in contrast with the 

 Lamarckian factors, and finally in the way that in the 

 view of Weismann it accounts for the origin of variations 

 as the result of the commingling of two differing parental 

 streams of germ-plasm. 



At first, for many reasons Weismann's theories did not 

 meet with general acceptance, but during recent years 

 there has been a marked return to many of his posi- 

 tions, mainly as the result of further cytological discov- 



23 



