362 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1915. 



I knew a breeder who had a chest containing 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 ex- 

 udation of drops of a tincture representing the sum of the charac- 

 teristics of the parent organism, but by a process of cell division, in 

 which numbers of these characters, or rather the elements upon 

 which they depend, are sorted out among the resulting gei'm cells 

 in an orderly fashion. Wliat 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 spermatozoon 

 IS obvious, but it seems to me imlikely that they are in any simple 

 or literal sense material particles. I suspect rather that their prop- 

 erties depend on some phenomenon of arrangement. However that 

 may be, analytical breeding proves that it is according to the dis- 

 tribution of these genetic factors, to use a noncommittal term, that 

 the characters of the offspring are decided. The first business of 

 experimental genetics is to determine their number and interactions, 

 and then to make an an^alysis of the 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 ques- 

 tion 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 crossbred in respect of the color of his eyes or the shape of his 

 mouth. Though we know nothing of the essential nature of these fac- 

 tors, 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 which animals and plants possess that we feel justi- 

 fied in the expectation that with continued analysis they will be 

 proved to be responsibl.e for most if not all of the differences by which 

 the varying individuals of any species are distinguished from each 

 other. I will not assert that the greater differences which character- 

 ize distinct species are due generally to such independent factors, but 

 that is the conclusion to which the available evidence points. All this 

 is now so well understood, and has been so often demonstrated and 

 expounded, that details of evidence are now superfluous. 



