BIRDS AND EVOLUTION 375 



To make more concrete what is meant by a germinal 

 disturbance expressing itself in a bodily variation, we may 

 take a particular case reported by Dr. Oscar Riddle (1917). 

 From an egg produced under the weakening influence of 

 " reproductive overwork " a female pigeon was hatched 

 which showed for a while a marked lack of control over the 

 voluntary movements of the head and body. This practically 

 disappeared in the adult, but when the affected female was 

 paired with two different males the derangement reappeared 

 in the progeny and continued through four generations. It 

 was a sort of hereditary ataxia, and behaved, with some 

 irregularities, as a Mendelian recessive. Thus out of 175 

 young pigeons reared to the age at which the disorder might 

 be exhibited, 119 were classed as normal and 46 as affected. 

 A subsequent study by Mathilda L. Koch and Oscar Riddle 

 (19 1 8) indicated that the disordered brains were suffering 

 from a chemical under-differentiation or immaturity. But 

 the immediate point is that a germinal defect, originating 

 in " reproductive overwork, expressed itself in a hereditary 

 brain disorder." The disorder is exhibited in all degrees 

 and has been inherited undiminished to the fifth generation. 



As an instance of what is meant by linkage of variations 

 we may refer to the close correlation which is seen in certain 

 strains of poultry between brachydactyly (reduction of the 

 size and number of joints in the toes) and the presence of 

 feathers on the instep region. The two peculiarities (and 

 probably coalescence of toes or syndactyly) seem to be linked 

 together, and are perhaps dependent on one and the same 

 factor, the nature of which is obscure (Danforth, 1919). 



The problem of the origin of the new is far from solution, 

 and the difficulty of the problem is emphasised by the range 

 of new possibilities which the concept of variation includes. 

 A variation may mean a feather less in the wing or twelve 

 feathers more in the tail ; it may mean a white blackbird or 

 a black sugar-bird ; it may mean a lengthening of the leg 

 or a strengthening of the wing-muscles ; it may be the 

 beginning of webs on the toes or of a gizzard in the stomach. 

 But a variation may be quite different from any of these — 



