234 GENETICS IN RELATION TO AGRICULTURE 
Sterility in Species Hybrids——A common phenomenon of species 
hybridization is the marked degree of sterility which is exhibited. This 
fact of sterility in speices hybrids has led certain investigators, par- 
ticularly Jeffrey, to lay great stress upon partial sterility as an evidence 
of hybrid character. This contention may be valid for a majority of 
cases, but obviously it would not follow even if all species hybrids dis- 
played partial sterility, that all cases of sterility are to be referred to 
hybridity. Specifically many instances are known for which simpler 
and more satisfactory explanations suffice. Thus Bateson has recorded 
a case of contabescence in the anthers of sweet peas which is strictly 
due to the presence of a definite factor for contabescence. The ratios 
obtained are approximately 3:1 ratios, contabescence being recessive, 
and moreover, the factor for contabescence is definitely linked with 
other factors, so that in every respect the factor analysis of this case of 
sterility is wholly satisfactory. In Drosophila similarly some cases of 
sterility are definitely referable to the action of specific factors which 
sometimes have effects so marked that strains homozygous for the 
factors in question cannot be maintained. This condition is well illus- 
trated by flies which are homozygous for the factor for rudimentary 
wings. Such flies are practically never fertile. Many other instances 
are known where slight effects on fertility result from factors which are 
intimately concerned in the expression of other characters. A some- 
what different type of sterility, but one which is also definitely established, 
is that which Bridges has reported for the males of Drosophila ampelophila 
which lack the Y-chromosome. The evidence upon which this case of 
sterility is based appears to be conclusive, and to demonstrate that while 
the male Drosophila lacking the Y-chromosome may develop a normal 
soma, it cannot produce functional germ cells. The sterility of wide 
crosses, however, appears to belong to a distinct category, an explanation 
for which we shall endeavor to give later on in this chapter. At this point 
weshall only take up some of the types of sterility displayed in such crosses. 
At the outset it may be well to note that the degree of sterility dis- 
played by hybrids varies from complete fertility to complete sterility. 
It is, therefore, readily apparent that sterility in hybridization as a 
means of species differentiation gives no natural divisions, but that 
arbitrary ones must be erected depending upon the degree of sterility 
displayed. Moreover, other factors such as those noted above com- 
plicate matters and render it extremely difficult to decide where to draw 
the line. Here again, therefore, the search for a universal species in- 
dicator has met with failure. From a genetic standpoint this is as it 
should be for it merely indicates that races of plants and animals display 
all degrees of genetic differences from simple differences in isolated factors 
to complex differences in entire series of factors. 
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