338 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



themselves, is not at all understood as yet. These congenital, 

 heritable variations may be, and usually are, small, but they 

 may also be large, and are then called "sports." They have 

 become especially familiar in recent years, under the name of 

 "mutations," in the condition of combinations or groups 

 occurring together, and thus making the individuals possess- 

 ing them readily distinguishable from the parent type. Recent 

 important discoveries in heredity have added to the interest 

 and importance of these congenital variations, and have thrown 

 a first faint light upon some of the conditions that attend their 

 origin. It is undoubtedly true that they are really the most 

 important actual beginnings of divergence among animals. 

 They are the building stones from which new species and better 

 adaptations are made. The importance of a knowledge of 

 variations on the part of animal and plant breeders is obvious. 

 It is especially important to distinguish between the heritable, 

 or congenital, and the non-heritable, or acquired, variations. 

 Only the first kind can be taken advantage of in the work of 

 making new races. 



Heredity. The extent and manner in which we inherit our 

 traits, both physical and mental, from our parents and an- 

 cestors has always been the subject of much speculation and 

 study. One of the greatest among students of heredity was 

 Francis Galton, only recently dead, whose studies, based 

 largely on human pedigrees, resulted in the formulation about 

 half a century ago of what is known as Galton' s "law of 

 ancestral inheritance." This states that on the average we 

 receive one-half our inheritance from our parents, one-fourth 

 from our grandparents, one-eighth from our great grand- 

 parents, one-sixteenth from our great, great grandparents, and 

 so on backward by halving fractions, the sum of them all being 

 one, or the total of our inherited qualities. 



Such a law, or generalization,- about heredity is of interest 

 and value, but it gives us little basis on which to predict the 

 specific character of the results of any mating. It applies 

 to beings and characteristics in masses, and to average in- 

 dividuals. And it does not say what half, or fourth, or eighth, 

 we shall get from any particular pair of ancestors. That is, 



