HEREDITY 319 



from generation to generation and by the mechanism of synapsis, 

 there is an interchange and mixing of chromatin materials, so that 

 the germ cells of the parent contain and transmit chromatin derived 

 from many lines of ancestry. 



This brief account leaves out much that is known concerning 

 mitosis, maturation, and fertilization, and is only in a generalized 

 form; there are many variations. Our account is sufficiently com- 

 plete to show that in the nucleus of the fertilized tgg are chromo- 

 somes, one-half the total number having been derived from each 

 parent, that include chromatin from previous generations. It also 

 shows that a chromosomal condition underlies sex differences, for 

 femaleness is associated with the presence of two X chromosomes, 

 and maleness with an X and a Y chromosome, or merely a single X 

 chromosome. It will be necessary to refer to these facts repeatedly 

 in connection with the study of inheritance; variations that are 

 significant will be discussed in their proper place. 



Inheritable Characters. With the essential facts concerning 

 the machinery of inheritance before us, we have next to examine 

 the nature and behavior of the characters of the adult animal as 

 they are passed from one generation to the next. The characters 

 of an individual may be grouped into three categories, the first 

 two of which are related or merely different treatments of the same 

 sorts of characters, (i) Characters that distinguish the animal as a 

 member of the species. For example, the common cat is recognized 

 as Felis domestictis because of the size, nature of the coat, number 

 of digits, shape of claws, etc. These features are transmitted from 

 one generation to the next, for the offspring of cats are always cats. 

 Species distinguishing characters appear to be fundamental proper- 

 ties of that particular type of protoplasm. The problem of the 

 origin of species will be discussed farther on in connection with the 

 subject of evolution. (2) Characters that distinguish the animal as a 

 variety within the species. For example, cats diflFer as to coat color, 

 depth of coat, and in other ways. Characters of this sort are trans- 



