16 INHERITANCE, FERTILITY, AND SEX IN PIGEONS. 



At the outset we would note that the second paragraph, referred to above, is 

 not an amphfication or explanation of the first paragraph; but each paragraph 

 tells of a different and main fact that had been learned concerning fertility. The 

 two succeeding paragraphs contain statements true alike for, and associated with, 

 both of the two main facts on fertility. 



The tabular statement of the second paragraph is of course merely a condensation 

 of a lengthy consideration already given in the introductory chapter concerning 

 the fertility of crosses in pigeons. We herewith reproduce, for the sake of clearness 

 and convenience, paragraphs covering the essential points: 



"The infertility of crosses is not to be confounded with sterility; it stands rather for 

 various degrees of specific incompatibility between the germ-cells of the cross-mated birds. 

 We do not know what this incompatibility ^ means, but it is certain that it is in no way 

 accounted for by external cavises, such as difficulties in copulation, or in the conjugation 

 of the sperm and ovum. In many cases the development of the egg is carried far enough — 

 to the formation of a blood-circle — to show that the sperm has entered the egg and fertil- 

 ized it. The development may halt at this point, or earlier, or be carried on to any later 

 stage, even to hatching. The young bird may die within a few hours, or live on, apparently 

 doing well, for several days, a week, or more, and then drop off as if life were a time-fuse 

 calculated to end at a definite moment. 



"The term of development often lies within such definite limits that, once ascertained, 

 we may know about what to expect in future trials with the same birds. The length of 

 the course to be run, although varying tvidely, is, on the average, cut shorter and shorter as the 

 crosses range from close allies to more distantly related species." 



Again: "The causes of infertility in crosses, or more correctly, the causes of fertility 

 in lower and lower degrees as the divergence between the crossed species increases, have 



yet to be investigated A single species may be fertile with all, or most of its 



congeners, and, in lower degrees, with members of other genera and even of other families." 



From his earlier work, then, the author learned that if pigeons most widely 

 separated phylogenetically were mated, the fertihty ("germ compatibility") there 

 was lowest and that no offspring could be had. If forms somewhat less widely 

 separated were chosen for matings, some germs were fertilizable, and in develop- 

 ment these would almost or quite invariably produce males. Forms still less 

 removed phylogenetically when crossed produced males predominantly; and so on 

 until closely related forms are reached which are fully fertile, and which throw the 

 sexes in approximately equal numbers. If, then, we allow birds to produce their 

 "strongest germs" (no reproductive overwork), but progressively cut down the 

 fertihty of these bij choosing consorts more and more distantly related, we thereby 

 increase the chances of producing male offspring,^ as long as any offspring may be 

 obtained. This is the gist of the earlier findings on the relationship of fertility 

 and sex. 



' One can perhaps ijrotitably remind liiinself here of the situation involved in anaphylaxis, immunity, etc. — 

 Editor. 



J. Loeb (Arch. f. Enluj.-k'nir, h , I'.il. l'7. I '"is Im.K m I lir ,\i i, imK \\\,\r cnisscs which he has made with lower 

 forms that the products ol |.:iii- ^i i lu- i-,i,.|nii -p .,,,, i,m ,ll\ :,ri ,,. , ]v„-.,u :(ihl nu-.r 1 1 ir .■!ins to develop abnormally. 

 O. Hertwig (Arch. f. Miki. An^ii . I'..!. .^J. llil:;. mill in innl- iliii -url, i,,i.ii;h ..r pui^onnus substances contained in 

 the sperm may be neutralized oiilcslroycd by prolun^i-ii licairm-nt wiih r:uliiiiii. Ciudlew.ski (Arch. f. Entwick'mech., 

 Bd. XXXIII, 1911) has called attention to the analogy between these features of fertilization and certain aspects 

 of immunity. — Editor. 



' This holds true also for matings (some genera) from which every egg, or almost every egg produced, is fertilized 

 and hatched. — Editor. 



