INHERITANCE IN POULTRY. 



By C. B. Davenport. 



Evolution proceeds by steps. These steps are measured by the character- 

 istics of organisms. When in the evolution of a race a characteristic is 

 added a progressive step is taken. When a characteristic drops out a retro- 

 gressive step is made. Since the characteristic is the unit of evolution, it 

 deserves careful study. The present work is a first study of the method 

 of inheritance of characteristics. 



A. STATEMENT OF PROBLEM. 



When by some abnormal process a single, fertilized egg develops into two 

 individuals they are, and continue throughout life to be, almost indistin- 

 guishable. This holds true even when the conditions of life of the two are 

 dissimilar. This case is exemplified by ' ' identical twins ' ' as they occur in 

 man,* The great similarity of such identical twins teaches that environ- 

 ment plays a small part in determining adult characteristics as compared 

 with heredity. Consequently more confidence can be felt that the results 

 of hybridization experiments are directly due to inheritance ; they are little 

 affected by varying environment. 



The children of the ordinary family are not identical in appearance, 

 although showing marked family traits. Certain characteristics may be 

 common, but others are peculiar to each individual child. This proves that 

 the fertilized eggs of the same two parents have not the same hereditary 

 potentialities. It indicates also that we cannot predict the characteristics 

 of the offspring from those of the parents. The proportion of qualities 

 derived from either one of the two parents will differ in different children, 

 or new qualities may appear. This is because the offspring do not inherit 

 from the visible part of the parents' bodies but from their hidden germ 

 cells or ' ' gametes. ' ' And the characteristics of the soma are never through- 

 out the same as those of the ripe gametes it carries. 



When the parents belong to different races having markedly dissimilar 

 characteristics there is not merely the question of dissimilarity of the off- 

 spring but of the inheritance of the antagonistic characteristics. Until 



* Gallon, F., 1883, pp. 216-243. Compare also for a critical study of resemblance in 

 twins, Thorndike, 1905. 



