166 GENERAL BIOLO<;Y 



a very rich milk but not a great quantity. Cows from another breed 

 may produce great quantities of less rich milk, while those of still an- 

 other breed living in the tropics may be more or less immune to heat 

 and tropical disease. If one wishes to bring cattle into a rather hot 

 clime, it will, therefore, be to one's advantage to obtain that breed which 

 will produce the greatest quantity of good rich milk and likewise be 

 able to withstand the great environmental change necessitated by re- 

 moval from a temperate or cold clime to one of great heat. 



We have already seen that the inheritable characters are contained 

 within the chromosomes. The definite factors, whatever they may be, 

 which carry the unit characters within the chromosomes are called 

 genes. 



From our knowledge, at the present moment, of the way the chro- 

 mosomes divide in cell division, and the way they throw off one-half 

 of their number during maturation just before 'fertilization so that fer- 

 tilization can again restore the regular number, we are led to believe 

 that no unit character can be inherited unless a gene from the father 

 and a gene from the mother unite in the chromosomes. 



We may say, for example, that all the unit characters which any 

 individual can possibly inherit are contained within the chromosomes of 

 the germplasm of its parents ; that each chromosome may contain thou- 

 sands of genes which may occur in any combination, the individual him- 

 self actually inheriting only those unit characters which happen to be 

 the result of the particular gene of paternal and maternal chromosomes 

 which met at the time of fertilization. 



To make this clear let us assume that a white and black guinea 

 pig are mated. The whiteness and blackness that we see, lie, of course, 

 in the somatoplasm ; but, in order that either color be inherited, there 

 must bf enes in the chromosomes of the germplasm which determine 

 the sor^atic character of whiteness and blackness. We know that if a 

 black guinea pig is mated with another black guinea pier, both of which 

 are in turn the offspring of an entire race of black animals, that only 

 black guinea pigs will be produced. However, if a black and white 

 animal mate, the offspring are really half-breeds, in regard to their 

 germ-cells, though their somatoplasm may show some variation in color. 

 We, therefore, assume, from the experimental evidence obtained through 

 breeding experiments of many kinds, that in order to produce a black 

 animal, both the paternal and maternal genes, which carry the determi- 

 nation of color, must have carried blackness. In the production of a 

 half-breed, one of the genes determining color must be white and the 

 other black. (Fig. 82.) 



In other words, two genes always meet to produce any character 

 sufficiently powerful to be carried on, in turn, through succeeding gen- 

 erations, and the character which is thus carried on and which shows 

 itself in the somatoplasm is called dominant. Blackness would thus be 



