GUINEA-PIGEONS CROSSED WITH COMMON PIGEONS. 89 



owl-rock is the exception). Furthermore, those that did begin to develop stopped 

 sooner, producing birds of a shorter term of life {G 1 x homer of 1903 is a possible 

 though not a probable exception). Of the 19 eggs of Series I which showed any 

 capacity for development, only one lived more than 19 days, as compared with 8 

 individuals from the 20 eggs of the other series which lived more than 7 months. 

 Thus it is here clear that the series of "least fertility" is also the series of "shortest- 

 lived" offspring. 



In Series I, moreover, it is evident that when the female is more nearly pure 

 (C. (abellaria), not obviously hybrid (owl-rock), that both eggs of the clutch are 

 rarely fertilizable; in none of the 12 clutches did both eggs show development.'* In 

 Series II, however, the 11 clutches (one clutch had but 1 egg) which showed devel- 

 opment showed it in both eggs of the clutch in 8 cases; 2 of the 3 exceptions are 

 first and last clutches of the season. The two series illustrate the fact that it is 

 when the developmental power of the germs is most strongly reduced that the breeding 

 evidence becomes clearest for the developmental {as apart from sexual) non-equivalence 

 of the two eggs of the pigeon's clutch. 



When owl-rock hybrids are mated (4 matings) with the males of either series, 

 the percentage of infertility is higher than when pure^ tabellaria or domestica 

 is used, and at the same time the young that develop from the hybrid unions have a 

 shorter term of life than have the young from females of pure strain. 



cTG 1. X owl-rock; 6 young lived — 3, 7 days; 1, 5 days; 1, S days; 1, 10 days; average, 7 days. 



d'G 2. X owl-rock; 4 young lived — 3, 7 days; 1, 10 days; average, S days. 



cf G 1. X homer; 8 young lived — 2, 1 day; 2, 9 to 10 days; 3, 14 to 19 days; 1, 15 months; average, 64 days. 



cTG 2. X homer; 6 young lived — 1, 10 months; 2, 12+ months; 3, 20 months; average, 470+ days. 



cf G 2. X domestica; 9 young lived — 6, 5 to 10 days; 1, 14 days; 1,11 months; 1, 30+ months; average, 142+ days. 



The data thus far considered lead, then, to the conclusions which are generally 

 supported by the data from other groups: (1) that fertility is a matter of all degrees; 

 (2) that proper crosses enable one to see and study the intergradations and flowing 

 levels of this important function; (3) that hybridization, even within the limits of 

 the breeds of common pigeons, makes their ova less compatible with the sperm of 

 one wild species; and (4) at the same time sets a sentence of a shorter term of life 

 upon the developments which proceed from their more compatible gametes. 



The non-equivalence of the two eggs of the clutch, particularly well seen in 

 cases where there can be but little development (as '\nG 1 x E 2), has also been 

 indicated. It remains to point out that these data further show that germs pro- 

 duced at the extremes of the seasons are weak in developmental power; fewer of 

 such eggs develop, and shorter lived offspring arise from them. This result can 

 here be best presented" in a table showing the outcome for the eggs of each month 

 of the year. Eliminating the record in Series II for the whole years in which no 

 fertile eggs were produced, the data are as shown in table 66. 



It will be seen that the "highest fertility" and "longest life" are both resident in 

 the eggs produced in May. The second for fertility is April; the second for longevity is 

 March. The lowest fertility is found in December, January, February, and August. 



° The egg of greater developmental strength and of male-producing tendency seems in domestic pigeons, even 

 of so-called "pure strains," to bear no definite relation to the order of the eggs in the clutch. This is as in hybrids 

 generally. 



° By "pure" of course only "relatively pure" is meant when speaking of common pigeons. 



'All such methods of grouping and treating data not entirely homogeneous are of course open to objection 

 but an estimate of the situation under analysis can be thus presented. 



