Review: Xenia in Fowls 



215 



tion (F2) according to the 'pea type' 

 of Mendel's law, in the proportion 3:1." 



EVIDENCE IS EQUIVOCAL 



As Walther, who critically reviews 

 the evidence, says, "The conclusion in 

 this last case does not fit the facts 

 very closely. What, in the first place, 

 is the 'pea type' of heredity? It is that 

 method in which, at the crossing of 

 organisms differing in one pair of iwn- 

 blending characters, the first hybrid 

 generation shows a complete conceal- 

 ment of one (the recessive) character 

 by the other (the dominant one), while 

 the second generation shows the pro- 

 portion o^" three dominants to one reces- 

 sive. And just what results did Holde- 

 fieiss actually secure? In the first gen- 

 eration (to which he considers the shells 

 of the eggs belong, from which the hy- 

 brid birds of the first generation will be 

 produced) both characters of the parents 

 (i. e., brown and white) appeared, 

 together with an intermediate form 

 which had not previously existed. For 

 the purpose of demonstrating his xenia 

 he picks out these intermediate eggs, 

 which are impossible in the 'pea type' 

 of heredity; for the purpose of demon- 

 strating the pea type of heredity, he 

 depends on the brown eggs, which he 

 himself admits are of no significance 

 for xenia, because likely not to have 

 been fertilized. In the second genera- 

 tion (by which he designates the eggs 

 laid by this first generation), instead 

 of the brown and white eggs in the 

 proportion of 3:1, which calculation 

 on the basis of Mendel's 'pea type' of 

 inheritance required, he got two brown, 

 119 brownish or intermediate, and forty 

 white ; whence he concludes that ' brown 

 showed itself to be the dominant 

 character.' " 



If this is the fate of Holdefleiss' third 

 proposition, it is evident that his second 

 one is on soft ground ; and the first one, 

 that xenia actually did exist, is far from 

 proved, considering the number of 

 possible explanations that might be 

 made. 



Domestic fowls are not an ideal 

 material for genetic experiments, be- 

 cause of the fact that they are all more 

 or less hybrid in origin, and their 

 appearance gives little idea of what 

 their germ-cells actually contain. Ar- 

 min von Tschermak,-'* professor of 

 physiology in the Veterinary High 

 School of Vienna, attempted to throw 

 light on the question by breeding 

 canaries and finches, closely related 

 species of wild birds which might be 

 expected to be purer in genealogy. 

 English or Harz canaries were mated 

 partly with males of their own kind, 

 and partly with males of five different 

 wild species. As a control, unmated 

 canaries were allowed to lay eggs that 

 had not been fertilized at all. The 

 results of the experiment, as described by 

 its author, may be summed up as follows : 



Changes in the form of the eggs, as a 

 result of paternal influence, could be 

 seen in only two cases, and these are 

 questionable; therefore no weight is 

 attached to them. 



CHANGES IN PATTERN 



There was no change in the ground 

 color of the egg shells. The pattern is 

 made up by two pigments; light brown 

 and dark brown. Alterations of the 

 light brown pigment were slight, but in 

 the direction of paternal influence. The 

 dark brown pigment, on the other hand, 

 showed modifications that cannot be 

 questioned. "They are so clear, that 

 an experienced eye can tell at a glance 

 what the paternal species is, in each 

 case. . . . As to the occurrence of 

 xenia in the coloring of bird's egg shells, 

 there is no longer room for any doubt. 



. . . The hybridization, in the cases 

 here analyzed, has exerted a specific 

 influence on pigment formation in the 

 egg shell." 



The unfertilized eggs of unmated 

 female canaries were then compared 

 with the fertilized eggs of female 

 canaries mated with males of the same 

 species. The former were found to be 

 slightly smaller, and almost lacking in 



3 See Biol. Centralblatt, 30 Band, 1910, S. 641-646 and Arch. f. die gesamte Physiologie, 148 

 Band, 1912, S. 367-395. The author is a brother of E. von Tschermak, the distinguished Austrian 

 plant-breeder who with Correns and De Vries was one of the rediscoverers of Mendel's work, 

 in 1900. 



