BIRDS AND EVOLUTION 387 



and more stable. On the other hand, there is increased out- 

 breeding or exogamy, which tends to promote variations and 

 divergence. There can be no doubt that alternations of 

 in-breeding and out-breeding have played an important part 

 in the evolution of birds, as in the case of other animal races, 

 and in human history as well. 



§ 10. Birds and Heredity 



Germinal Continuity. — The largest fact in regard to 

 heredity (the genetic relation between successive generations) 

 is that like tends to beget like. This is due to " germinal 

 continuity." Let us suppose a fertilised egg-cell with the 

 implicit qualities abcxyz ; it divides and redivides to build 

 up a body in whose cells there is division of labour, some 

 showing more of the quality a, others more of the quality of 

 Zy and so on. But while this body-making is going on, 

 certain cells do not share in it, but remain undifferentiated, 

 preserving intact and uniformly the potential qualities, 

 abcxyz. These are the reproductive cells — let us say the 

 eggs. And if one of these is liberated, with the original 

 qualities unchanged, it will naturally develop into a creature 

 like the parent. In each development, as Weismann said, 

 a portion of the specific germ-plasm contained in the parent 

 egg-cell is not used up in the construction of the body of the 

 offspring, but is reserved unchanged for the formation of the 

 germ-cells of the following generation. This is the fact of 

 germinal continuity and the reason why like tends to beget 

 like. 



In the early days of embryonic development in birds 

 there is a segregation of the germ-cells that form the 

 beginning of the reproductive organs. The distinction 

 between body-forming cells and germ-cells is well defined. 

 In other words, the germinal continuity is clear. We no 

 longer ask whether the hen makes the egg or the egg the hen. 

 The true statement is that the fertilised egg develops into the 

 bird and the germ-cells thereof — egg-cells or sperm-cells 

 as the case may be, 



