Reactions and Pkoducts in Interspecific Crosses 169 



pattern, which is not in experience dissociated in this combination. The chief 

 distinction is the form-index, which permits of sharp differentiation even by 

 inspection and separation of the F, extracted groups. 



SUMMARY AND DISCUSSION OF THE CROSSING OF SPECIES. 



It has been shown in these two chapters that organisms from nature that have 

 the taxonomic value of species, some of which have been distinguished as such 

 for more than a half century, can be crossed and give fertile progeny and types 

 of reactions that are interesting. The first question that naturally arises is, are 

 these forms true species ? But what are true species ? 



It has been shown in Chapter II that each of the forms used here have precise 

 habitat relations and differences, are at all stages in life distinguished by con- 

 stant characters of color, pattern, form, and the many specific methods of 

 reaction characteristic of each, between which species there are no present inter- 

 grades. Nevertheless, they cross and produce fertile progeny to greater or less 

 degrees, and in this transgress one of the oldest criteria of distinctness. It is 

 now certain that there is no certain line of separation in this respect, and that 

 from complete fertility one passes by all sorts of transitions to complete infertil- 

 ity. What constitutes fertility or infertility remains for the future to deter- 

 mine. Some data of significance is gathered from these experiments. 



The problem of fertility of species hybrids presents two aspects : ( 1 ) as to the 

 factors productive of limitation of breeding in the initial cross; (2) of the fac- 

 tors productive of sterility or nonsterility in differing degrees in the F^ array 

 in breeding. In Darwin's time, and even before it was known that the sterility 

 of hybrids between species was a complicated relation and the work of Gartner, 

 Kolreuter, and others, as well as his own experiences, forced Darwin to admit 

 that " we have conclusive evidence that the sterility of species crosses must be 

 due to some principle, quite independent of natural selection," " and from the 

 laws governing the various grades of sterility being so uniform throughout the 

 animal and vegetable kingdoms, we may infer that the cause, whatever it may be, 

 is the same or nearly the same in all cases." Thus Darwin clearly recognized 

 that sterility, of which so much could be made in discussion of species and their 

 origin and its advantage to incipient species, rested upon other principles, and 

 in the end he concludes that " the primary cause of the sterility of crossed species 

 is confined to differences in their sexual elements." 



Sterility, in the broad use of the term, comprises several different aspects of 

 the fact of nonproduction of progeny in a given mating. Commonest are the 

 instances of mechanical inhibition of breeding or copulation, which is usually 

 due to structural relations or conditions whose origin is unknown ; second, there 

 are the numerous series of instances where the gametes are able to unite and ini- 

 tiate developments, which, however, do not progress beyond a certain stage in 

 ontogeny, a stage which seems in many instances to be a constant point for the 

 particular combination; third, the array of numerous examples of the produc- 

 tion of an Fi hybrid, frequently with unusual personal strength and vigor, but 

 inability to produce gametes that are capable of either fertilization or develop- 

 ment or both. Between these three modal points there are many intermediate 

 conditions described in the voluminous literature of hybridization. 

 12 



