l80 DAVENPORT 



preceded by the union of bits of the germ-plasm coming from 

 different parts of the same motor stream, or even from two 

 different streams, recently separated. This is union of sex cells, 

 or gametes, in sexual reproduction. As every part of the germ- 

 plasm is undergoing change, such unions bring together more 

 or less dissimilar germ-plasms. Of the effect of such unions, 

 we get some notion b}^ studying the somas, or offspring that 

 result. If the uniting germ-plasms are quite dissimilar the 

 behavior of the union, as revealed in the soma of the hybrid 

 offspring, is correspondingly interesting and significant. 



A second sort of movement in the germ-plasm is molecular. 

 Under certain, still unknown, conditions the composition of the 

 germ-plasm changes and such changes reveal themselves to us 

 as sports or mutations. These occur so irregularly and uncom- 

 monly that their study is difficult ; but it is one of the aims of 

 experimental work in evolution to initate and control such 

 changes. 



This evening I propose to speak only of the first kind of 

 changes in the germ-plasm — : namely, those due to combinations 

 of two dissimilar sorts — and the effect of such combinations on 

 the stream of germ-plasm that issues from the union. The laws 

 of such combinations and their transmission constitute the science 

 of heredity. 



In analyzing the phenomena of heredity we must first con- 

 ceive that the germ-plasm is composed of many units, such as 

 Darwin named pangenes and Weismann determinants. Our 

 knowledge of such units is confined to their representations in 

 the soma — the unit-characters oi organisms. 



In studying heredity, we focus attention on the behavior of 

 the unit-characters of the soma as the index of the changes in 

 the germ-plasm. Practically, one breeds together two indivi- 

 duals having contrasted characters and observes the correspond- 

 ing characters in the next somatic generation. 



The results of such experiments are in the highest degree 

 interesting and important. They are not yet predictable ; 

 indeed, they do not all follow a single law, but in the midst of the 

 diversity of the results a unity is diml}^ visible. I propose now 

 to cite typical examples of such inheritance in contrasted charac- 

 ters, illustrating the different modes of inheritance. 



