INTIi'ilH 'I "HUM . 



The term of development to which we have just mad.- n-feren-.-..fien li.- within 

 such definite limits that, once ascertained, \ u - may know about ul, in 



future trials with the same birds. In the pi-eons I find that the length of t|,, 

 to be run, although varying widely, is, .m the average, cut >h.,rier ami -I,. 

 the crosses range from close allies to more distantly relate.! 



The causes of infertility in crosses or, more correctly, th- of fertility 



in lower and lower degrees as the divergence between the crossed specie,, m, 

 have yet to be investigated. What connection, if any, do such phenom.-na 1 

 with the formation of so-called "pure" germs in hybrid^'.' If 'purity" i< a thing 

 of degrees, as it most certainly is, do the degree^ rifle <>r fall with fertility'.' Further- 

 more, if degrees arc so numerous as to appear to flow together, can a f.-\\ . 

 approximate "purity" be claimed as a law? Do higher degrees differ in kind I 

 lower, and so justify the assumptions of discontinuity," mutation If 



two wide-apart degrees are found with no intergrades. hou , conclude that 



they are both "sports" without genetic connection. 



In dealing with such questions, the first, requisite is material >uited t.. 

 definite answers. In this respect the pigeon group is an ideal one. for it i> e..mp' 

 of so large a number of closely graded species that it must afford some pooh 

 evidence of "flowing degrees," if such exist, and at the same time give u.- t In- 

 directions of rise and fall in a considerable number of well-defined differential 

 characters. For the experimental side of the study we require not only sper 

 will cross with continued fertility, but also species that will cross with varying 

 degrees of fertility. Both arc essential and both are represented in this group in 

 exceptional abundance. 



As there are between 400 and 500 distinct species of wild pigeons, and a- nearly 

 every species may be presumed to be fertile with at least, one other species, the 

 number and variety of fertile crosses possible must, at the lowest estimate, be quite 

 large. But the number is probably many times larger, for a single 

 fertile with all or most of its congeners and, in lower degrees, with members of other 

 genera and even of other families. 



The common ring-dove, for example, is fertile not only with some .! 

 other species of its genus (Streptopelia, collared doves!, but also in lower 

 degrees with the typical turtle-doves, as two successful tests one with the 

 European turtle-dove (Turtur turtur} and another with the Surate turtle-d 

 (Spilopelia suratensis) of Burma and the Malay Peninsula sufficiently att. 

 In still lower degrees it is fertile with the mourning-dove ('/.< nanlnm cnrl 

 with the white-winged pigeon (Melo]>cli<i lciic<i/>ttr<i t, and with many i 

 domestic pigeons. 



How much farther the fertility of this species may extend remains to be tested. 

 Some other ring-doves, as the wild species of China and Japan ^Shrptnpiliu dour. 

 and the red ring-dove (St. humilis) of the same countries, give evidence of equally 

 strong fertility in crosses. The Oriental or Japanese turtle-dove Turf*. 

 promises an even wider range of fertility, as I have obtained one fertile hybrid 

 between it and Columba livia domestica. This is, I believe, the most remarkable 

 case of fertility hitherto recorded. If would certainly be difficult to match it in 

 any other group of the higher animals. The offspring of this hybrid exhibit to 



11 Also later found to be fertile with the .l;ip:inr.-e turtle '"R. 



