ZOOLOGY 453 



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 various 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 can not 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 inheritance, it demon- 

 strates 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 positions, mainly as the result of further cytolog- 

 ical discoveries, and of the formulation of Mendel's law and of De 

 Vries's mutation theory. The first-named law was propounded by 

 Gregor Mendel on the basis of extensive experiments upon plants con- 

 ducted during many years, from 1860 on, in the obscurity of his mon- 

 astery garden at Altbrunn, in Germany. It was rescued from oblivion 

 by De Vries who found it buried in a mass of literature and brought 

 it to light when he published his renowned mutation theory in 1901. 

 Mendelian phenomena of inheritance, confirmed and extended by 

 numerous workers with plants and animals, prove that in many cases 

 portions of streams of germ-plasm that combine to form the hereditary 

 content of organisms may retain their individuality during embryonic 

 and later development, and that they may emerge in their original purity 

 when the germ-cells destined to form a later generation undergo the 

 preparatory processes called maturation. They demonstrate also the 

 apparent chance nature of the phenomena of inheritance. I think the 

 most striking and significant result in this field is the proof that a 

 particular chromosome or chromatin mass determines a particular 

 character of an adult organism, which is quite a different matter 

 from the reference of all the hereditary characters to all of the chro- 



