BIRDS AND EVOLUTION 389 



The most important point in regard to the factors of 

 unit-characters is that they behave in heredity as if they 

 were definite entities, Uke the radicals in chemistry, which 

 can be shuffled about and distributed to the offspring in 

 some degree independently of one another. 



Different Modes of Inheritance. — There are various 

 possible results when two birds are crossed. 



(a) The offspring of two pure-bred birds which are very 

 like one another may be practically indistinguishable from 

 the parents. This is complete hereditary resemblance. 



(b) The offspring of two divergent birds (within the same 

 species, of course) may be something distinctly novel. It 

 may show a mutation. This is the other extreme. 



Between these two extremes there are three possibilities. 



(c) The offspring may be a blend as regards certain 

 characters between the two parents ; and among domesti- 

 cated birds the occurrence of what looks hke blending is not 

 uncommon. Whether what looks like blending inheritance 

 is really such cannot be determined without data in regard 

 to the next generation. One requires to know whether all 

 the grand-offspring show the appearance of blending that 

 the offspring exhibited. 



Dr. Ernest Warren (1914) has described two hybrids 

 between two kinds of cockatoos, belonging to different 

 genera — Cacatua galerita (male) and Licmetis nasica (female). 

 The hybrids seemed to show some blending of the characters 

 of the two parents. Out of ten characters the hybrids were 

 nearer Cacatua in five, nearer Licmetis in one, and almost 

 exactly intermediate in four. In every character examined, 

 with the possible exception of the coloured and non-coloured 

 lores (the space between the bill and the eye), there was a 

 very obvious blending of the paternal and maternal character- 

 istics. But we require to know more about such cases. 

 When a character owes its development to the co-operation 

 of several " factors," the offspring might have some of these 

 from the mother's side and the rest from the father's side, 

 so that the character appeared as a blend although no 

 maternal factor was affected by a corresponding paternal 



