136 STRUCTURAL HYBRIDS 



phylogeny. Thus it may happen that a cross between two dis- 

 similar zygotes, if these were themselves hybrids, is constituted of 

 two gametes that are identical ; speaking in mendehan terms, 

 Aabb and aaBb differ, but may give homozygous offspring aahh. 

 Hence it may happen that a first cross between two taxonomic 

 species in (Enothera is less hybrid than its parents, and derivatives 

 of such hybrids have been obtained which constitute pure Unes in 

 the strictest sense (Ch. IX). 



A hybrid (or heterozygote) must therefore be defined as a zygote 

 which either arises from the union of dissimilar gametes, or gives rise 

 to dissimilar gametes (regularty, i.e., apart from mutation). For 

 most purposes it will be found that these alternatives lead to the 

 same result, and the second can be ignored except in relation to 

 polyploids, in which hybridity requires special consideration, 

 l^olyploids have chromosome sets corresponding with those of more 

 than two ancestral gametes, and are capable of functional relations 

 with one another, so that a simple definition of hybridity is there- 

 fore inapplicable to them. 



Hybrids have very generally the property of hj^brid vigour or 

 " heterosis " which is sometimes due to the normal allelomorphs of 

 depressive factors accumulated in their more homozygous parents 

 and sometimes, perhaps, to an inherent virtue in the possession of a 

 single dose of material in which the parents differ. Hybrids, both 

 in plants and animals, are liable to abnormality, which is due to 

 their having a new and untried hereditary complement. This 

 liabilit}^ is found in an exaggerated degree in one sex (the 

 heterozygous sex) where the sexes are differentiated (Haldane, 

 1922) because the normal chromosome set of one species is not 

 wholly represented in the hybrid and the deficiency may not be 

 made up from the other. It may be said that the heterozygous 

 sex in an animal hybrid is not strictly an F^ hybrid at all, since it 

 differs from the hybrid of the homozygous sex through the hybridity 

 of one of its parents. Its properties are not therefore the char- 

 acteristic properties of F^ hybrids. They are incidental to special 

 genetic conditions. 



The characteristic properties of hybrids depend, therefore, not on 

 the properties of the parents, but on the differences between these 



