CHAPTER XVII. 



ON THE DIVISIBILITY OF CHARACTERS. 



Specific characters, bj' hybridizing, can be divided, subdivided, etc., until it is 

 seen that two different characters are brought to an equalit}'. Where two species 

 have different characters, one can get every degree of splitting or division. 



If the characters of the crossed birds are the same, then the resulting hybrid has 

 the full unreduced character. 



AN EXHIBITION OF HYBRIDS' SHOWING THE DIVISIBILITY OF CHARACTERS. 



Professor Brooks has defined heredity as "the resemblance of child to parent, 

 of offspring to ancestor; while the difference between child and parent is called 

 variation."- 



Most of us feel that this definition covers the ground. In the investigation of 

 heredity, or in a discussion of it, it is, however, extremely difficult to limit ourselves 

 to single things; and the only way to do it — if there is any waj^ to do it — is to get 

 some particular objects before our eyes and make the effort to see for ourselves what 

 is to be seen. At this time I shall content myself with exhibiting a few pictures and 

 a few living birds to illustrate \\hat happens in the crossing of different species of 

 pigeons. 



In this field, as in most other fields, a single fact, or what we can learn from a 

 single fact, is simply illustrative of what can be found in the rest of the field. The 

 first hybrids that I shall discuss are those derived from the common ring-dove and 

 the nearly extinct passenger-pigeon. Both sexes of the passenger-pigeon have 

 been carefully represented in color drawings (the male in pi. 28 and the female in 

 pi. 29). The main distinction between the sexes is that the female has a duller 

 color, has more brown, is rather larger, and has more numerous spots on the wing. 

 The male has apparently lost a certain number of these spots and has reduced the 

 size of the rest, some of them being so minute that they can scarcely be seen. Some 

 of them are concealed under the wing-coverts, where, of course, they can have very 

 little ornamental importance. ^ The blond ring-dove may also be seen in color (pi. 8) ; 

 the neck-mark or ring of an adult female is better shown in pi. 31; the ring of a 

 Juvenal ring-dove is also shown in pi. 31. 



The hybrids from the passenger x ring-dove cross, of which some 8 or 10 have 

 been obtained, were all males. The father of all these hybrids was a passenger- 

 pigeon and the mother a ring-dove. The hybrids (one shown in pi. 30) will 

 perhaps best be compared with the male of the paternal species. An examina- 

 tion of the neck-mark attests that in this respect the hybrid stands as nearly 

 intermediate between its two parents as is possible. The color of this region is 

 lighter in the hybrid than in the passenger-pigeon, and that is of course in the 

 direction of the ring-dove. The passenger-pigeon shows a plain iridescence on the 

 side of the neck. In the hybrid there is a slight iridescence; and it has in addition 

 the differentiation of the feathers of the ring that carries it beyond the passenger- 



1 The manuscript (SS 11) used in this chapter is a stenographic report, partly corrected by the author, of a lecture 

 at Woods Hole, July 19, 1906. The editor has adjusted the manuscript to a place in this volume. 



2 W. K. Brooks, Proc. Amer. Phil. Soc, No. 182, April 1906, p. 70. 



^ See text-figs. 5 to 7, Vol. I. 



