174 ORTHOGENETIC EVOLUTION IN PIGEONS. 



"It is remarkable that a character of one parent that was entirely lost with the 

 juvcnal feathers should reappear in one of her hybrid offspring, but not in the other. 

 Z-ZN 2, the mate, has no such marks. The inheritance in one offspring, however, 

 shows that the mother still carries these spots potentially in some of her germs. 



"This white mark is, however, only an 'enlarged copy' of the obscure mid- 

 streak — pale reddish or buff in color — which generally can be seen on close examina- 

 tion in the breast-feathers of the more typical young zenaidas." 



GENERAL CONSIDERATIONS. 



The mutation hypothesis offers an easy escape from the difficulties presented 

 in getting from one color-pattern to another. Like the creation hypothesis, muta- 

 tion just covers each problem with an infallible and inscrutable saltation. 



As Darwin 46 has well said, this view forces one " to admit that these great and sud- 

 den transformations have left no trace of their action on the embryo. To admit all this 

 is, as it seems to me, to enter the realms of miracle, and to leave those of science." 



If refuge from this truth-telling criticism be sought in minute invisible premu- 

 tations, assumed to occur in the primordial germ-cells, then the "transformations" 

 would shrink to the dimensions of ordinary heritable germ-variations and be quite 

 indistinguishable. Mutations would thus merge in minute variations and leave 

 only an empty name for an explanation. 



Mutation stands for gaps and no bridges — jumping the very problem to be solved. 

 Its discontinuities are unpredictable breaks in the lines of derivation, precluding 

 both foresight and hindsight. 



Specific characters are not separate units, but qualities belonging to the whole 

 organism. When albinism occurs we see that it tends always to be wholesale; that is, 

 the whole color is bleached. If it comes into greater prominence in one region than 

 elsewhere, we do not find a sharp boundary-line, but rather a gradual reduction or 

 increase in this or that direction. The Zenaida mutation shows this beautifully. 



In the same way, if we find a particular color-mark in one feather — say the 

 light edge, or the dark center, the transverse bars, the white dots (Geopelia cuneata) — 

 we find this mark tending to appear in all feathers; or we find it in derivative 

 forms, or possibly in faded or weak forms. The loss of spots in pigeons, the turning 

 to gray, etc., are all phenomena of the bird as a whole. We accept neither the 

 monotone germ-plasm nor the polymikt germ-plasm of Weismann. 



46 Origin of Species, Chapter VII, p. 241. 



Explanation of Plate 86. 

 A, B, C. Later offspring (1910) of the original Zenaida mutant (21). 



A and B. Front and rear views of two good mutants (at ends) and one weak or "crescent" mutant (center) in Juvenal 

 plumage. (Of complex derivation, hatched Hill, reaching to No. 21 through Z-ZN 1. — Ed.) 



C. Two second-generation (1910) mutants, 2Z-3ZN 7 and 8, age 6 weeks. The breast has guinea-marks well-defined, 



but rather smaller than in the best mutant offspring. The head had very small or narrow spots. The 

 wing-coverts were white-edged, but without the triangular spots, except very weak marks at the bend. 



D, E. Zenaida mutant offspring of 1915 and 1916. 



D. Fourth-generation normal (nearly) and mutant offspring (B227 and B229, hatched July 30, 1915). Dam, 3Z- 



6ZN 1 (see chap. 10, vol. ii) ; sire, 876, hatched 1912 (from complex Zenaida X Zenaidura parents). (Photo- 

 graphed Aug. 1915 by C. W. Palmer.— Ed.) 



E. Fifth generation of mutant offspring, Nos. 82 and 83 (smaller pair); hatched June 30 and July 1, 1916. Dam, 



916 (1915 sister of birds of fig. 4, above); sire, 876 (1912, from complex Zenaida X Zenaidura hybrids of 

 incomplete pedigree). The two larger birds (at ends) are 1916 offspring, of the fourth generation, hatched 

 May 23, 1916, from same parents as 1915 young shown in figure 4. These birds had probably molted some 

 feathers. Both are fairly strong mutants. (Photographed July 11, 1916, by C. W. Palmer. — Ed.) 



