236 SECRETS OF ANIMAL LIFE 
and so on. The doubt left in one’s mind is whether 
the auxiliary hypotheses which make difficult cases 
conformable are triumphs of human ingenuity or 
indices of the subtlety of life. When we are led 
by other authorities to contemplate the disintegra- 
tion or fractionization of the factors of certain unit 
characters, our faith is strained by what seems so 
like a contradiction in terms. But the scientific 
position is to experiment and see. Very interesting 
on another line are the experiments of T. H. Mor- 
gan’ and his collaborators on the fruit-fly Droso- 
phila, showing that “ every character is the realized 
result of the reaction of hereditary factors with each 
other and with their environment.” Flies of a 
race with a peculiar hereditary abnormality will 
develop normally if raised in a dry environment, 
but the presence within them of the factor for 
abnormality may be demonstrated by rearing their 
offspring in moisture. In other words, the ex- 
pression of even Mendelian characters requires an 
appropriate nurture. 
Professor Wilson lays stress on the fact that 
theoretical difficulties need not hinder the breeder 
from utilizing the Ariadne thread which Mendel has 
put into his hand. For it is becoming increasingly 
clear that Mendelism can enable a breeder or 
cultivator to reach his desired result more surely, 
more rapidly, and more economically. His new 
knowledge shows him how desirable qualities of the 
unit character type can be grafted on to a stock, 
*The Mechanism of Mendelian Heredity. Constable, 1915. 
