CHAPTER VI. 



EFFECT OF SEASON OF HATCHING UPON DISTRIBUTION OF FERTILITY 



AMONG THE OFFSPRING OF AN OVERWORKED PAIR OF 



JAPANESE TURTLE DOVES. 1 



In earlier chapters the function of fertility has been examined in its relation 

 to sex and to longevity, and with reference to its dependence upon age, seasons, 

 inbreeding, width of cross, and the experimentally induced "overwork." Some 

 later chapters, too, bear in part upon those topics. It is now, however, of interest 

 to inquire whether the reduced fertility or "weakness" of germs is continued into 

 the germs of the next generation, i.e., whether this reduction is of significance in 

 heredity as well as in the individual development. The demonstration of the rela- 

 tion which fertility bears to the development of sex is sufficient definitely to give 

 it a genetic value, but we may now examine more specifically some data that have 

 to do with the question of the continuance in the offspring of the lower and higher 

 levels of fertility which have been seen to correspond to season. 



In the preceding chapter the continuance of low levels ("mutations") of certain 

 characteristics, which were likewise originally induced by extreme age and inbreed- 

 ing, have been considered. Near the conclusion of that presentation the inbred 

 family (table 27) which now becomes the subject for special treatment was referred 

 to briefly. 



The relationship of the two birds chosen as parents for the family in question, 

 and the earlier performance of these same birds (table 26A) was, for us, a sufficient 

 guarantee that some weak progeny could be secured from this mating at the extreme 

 end of the season; and if a considerable amount of "overwork" could be induced, 

 sufficiently large numbers of young might be secured in a single season to supply 

 material for an additional test a further comparative test to learn whether the 

 germs of birds which themselves arose from germs of reduced fertility will in turn 

 reveal reduced fertility; - and whether birds from the same parents, proceeding from 

 stronger, more fertile germs, will in turn form germs of higher developmental power. 



The investigation of the fertility of the young of this pair of birds bred in 1911 

 has supplied a body of very interesting data. It is perhaps still too early to write 

 the complete story of their relative fertility, since some of the family are still alive. 

 It is nevertheless already clear that those birds which developed from the "stronger 

 germs" of the earlier part of the season of 1911 in their turn have produced germs 

 of greater developmental power than have been produced by their brothers and 

 sisters hatched from eggs laid later in the season under stress of reproductive 

 overwork. 



1 This chapter was written wholly l>y the editor, and is based upon a breeding study which was also carried out 

 by him (see next foot-note I. The study is based on a mating (table 2(5) arranged and followed during 1910 by the 

 author. 



- An early examination of the author's manuscripts by the editor made it clear that although there was much 

 evidence bearing on this point (of a continuance of reduced fertility from an induced low fertility and an association 

 of weak germs with femaleness), the form of the records would make it difficult or impossible for any one other than 

 the author to present the "whole" of the fertility tests for all of the members of any large family treated in the author's 

 records. It was therefore decided to make of this series of 1911 the series now about to be described -a new and 

 additional test of the matter. By making the records himself, and having this specific point in mind in making the 

 matings for the fertility tests, the editor is able to give a fairly full account of this test case. 



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