290 



SCIENCE 



lN. S. Vol. XL. No. 1026 



The common speech uses expressions such 

 as consanguinity, pure-blooded, half-blood, 

 and the like, which call up a misleading 

 picture to the mind. Blood is in some re- 

 spects a fluid, and thus it is supposed that 

 this fluid can be both quantitatively and 

 qualitatively diluted with other bloods, just 

 as treacle can be diluted with water. Blood 

 in primitive physiology being the peculiar 

 vehicle of life, at once its essence and its 

 corporeal abode, these ideas of dilution and 

 compounding of characters in the com- 

 mingling of bloods inevitably suggest that 

 the ingredients of the mixture once com- 

 bined are inseparable, that they can be 

 brought together in any relative amounts, 

 •and in short that in heredity we are con- 

 cerned mainly with a quantitative problem. 

 Truer notions of genetic physiology are 

 given by the Hebrew expression "seed." 

 If we speak of a man as "of the blood- 

 royal" we think at once of plebeian dilu- 

 tion, and we wonder how much of the royal 

 fluid is likely to be "in his veins"; but if 

 we say he is "of the seed of Abraham" we 

 feel something of the permanence and in- 

 destructibility of that germ which can be 

 divided and scattered among all nations, 

 but remains recognizable in type and char- 

 acteristics after 4,000 years. 



I know a breeder who had a chest con- 

 taining bottles of colored liquids by which 

 he used to illustrate the relationships of 

 his dogs, pouring from one to another and 

 titrating them quantitatively to illustrate 

 their pedigrees. Galton was beset by the 

 same kind of mistake when he promulgated 

 his "Law of Ancestral Heredity." With 

 modern research all this has been cleared 

 away. The allotment of characteristics 

 among offspring is not accomplished by the 

 exudation of drops of a tincture represent- 

 ing the sum of the characteristics of the 

 parent organism, but by a process of cell- 

 division, in which numbers of these char- 



acters, or rather the elements upon which 

 they depend, are sorted out among the re- 

 sulting germ-cells in an orderly fashion. 

 What these elements, or factors as we call 

 them, are we do not know. That they are 

 in some way directly transmitted by the 

 material of the ovum and of the sperma- 

 tozoon is obvious, but it seems to me un- 

 likely that they are in any simple or literal 

 sense material particles. I suspect rather 

 that their properties depend on some phe- 

 nomenon of arrangement. However that 

 may be, analytical breeding proves that it 

 is according to the distribution of these 

 genetic factors, to use a non-committal 

 term, that the characters of the offspring 

 are decided. The first business of experi- 

 mental genetics is to determine their num- 

 ber and interactions, and then to make an 

 analysis of th,e various types of life. 



Now the ordinary genealogical trees, such 

 as those which the stud-books provide in 

 the case of the domestic animals, or the 

 Heralds' College provides in the case of 

 man, tell nothing of all this. Such methods 

 of depicting descent can not even show the 

 one thing they are devised to show — purity 

 of "blood." For at last we know the 

 physiological meaning of that expression. 

 An organism is pure-bred when it has been 

 formed by the union in fertilization of two 

 germ-cells which are alike in the factors 

 they bear; and since the factors for the 

 several characteristics are independent of 

 each other, this question of purity must be 

 separately considered for each of them. 

 A man, for example, may be pure-bred in 

 respect of his musical ability and cross-bred 

 in respect of the color of his eyes or the 

 shape of his mouth. Though we know 

 nothing of the essential nature of these 

 factors, we know a good deal of their 

 powers. They may confer height, color, 

 shape, instincts, powers both of mind and 

 body; indeed, so many of the attributes 



