90 Disease and Immunity 



greater efforts on the part oi this living substance 

 in making the proper union ani^divisions. 



One of the phenomena of hybridization is that 

 the hybrids are commonly more vigorous than 

 either parent. This extra vigor does not come 

 from nowhere out of nothing. Vigor means phys- 

 ical power of some kind, and powers are devel- 

 oped in living organisms by exercising those 

 previously in existence, and in no other manner. 

 Primarily, the vigor of hybrids is increased 

 power of growth, and increased growth means 

 increased power of cell division. We can trace 

 this increased power in hybrids directly to the 

 extra efforts (increased exercise) which the im- 

 like germ cells had to make to form a proper 

 union and then proper divisions of the mixture of 

 differently organized substances. 



Another thing observed in hybridization is that 

 the extent to which the hybrid is more vigorous 

 than the parents increases with the wideness of 

 the cross up to a certain point, after which there 

 is a decrease of vigor in offspring. In wide 

 crosses the offspring are abnormally weak, and 

 if the cross is still wider there is a failure to de- 

 velop. In some wide crosses, as in the mule, 



