CHAPTER I. 



INTRODUCTORY. 1 



All domestic breeds of pigeons, of which there :ire probably not |e-> than 

 distinct kinds, are fertile when crossed and the offspring of . il-o fertile. 



The same holds true, so far as known, het \\een the domestic races and the \\ild 

 rock-pigeons, and this fact is generally held to be a strong indication of common 

 ancestry. 



In the case of the wild species of pigeons, of which there arc nearly .">(Mi 

 are very often infertile, and fertile hybrids are so rare that Dar\\in c.>uld not find 

 "a single well-ascertained instance of hybrids between two true -p cic - of pi^, 

 being fertile inter se, or even when crossed with one of their pure parent- " An in inl- 

 and Plants, I, p. 237). The records since Darwin's time have not furni.-hed the 

 "instance" he vainly sought for. 



Even if fertile hybrids were sometimes obtainable, the pos>ibility of establishing 

 an "intermediate" race between two wild species would pre.-umably 1 

 doubtful, since such a result as Darwin points out is extremely diflicult e\en in 

 the case of domestic breeds where the hybrids are perfectly fertile. Darur 

 "Certainly, a breed intermediate between two very distinct breeds could no' 

 got without extreme care and long-continued selection; nor can I find a caw on 

 record of a permanent race having been thus formed." (Origin of Specie-, ."iih 

 p. 33.) 



The results of experiments with wild species since Darwin s time have : 

 very meager; with comparatively few exceptions, domestic animals and cultivate.! 

 plants have been preferred to wild species. The very important advantaue- of 

 natural species for such work have either been entirely overlooked or greatly 

 underestimated. 



The difficulty with domestic forms is that they have a complex ance>try uhicli 

 we can never hope to unravel. These forms are themselves the products of a 

 most tangled series of hybridizations, 2 and hence they give, for the imt part. 

 unpredictable results "reversions," "regressions," "mutati 'sport.- 



genus omne. With such material each individual, as Gallon puts it. "should be 

 viewed as the fulfillment of only one out of an indefinite number of mutual 1 

 sive possibilities." The unfulfilled possibilities lurk about in so-called 

 characters that may sleep for generations and then appear :i 

 no law except "the law of large numbers, 11 which virtually says 

 explanation for the individual phenomenon; be satisfied to know that 

 many phenomena which always follow that wonderful curvi 

 defined itself by Newton's binomial theorem, and which i> 

 to life insurance, pension laws, etc." 



Without demurring to the many interesting applications ot 

 numbers," it must not be forgotten that the individual phen 



'This paper was written (1904-5) under > "Hybridi "I \V,M BpeCM of ; 



2 Mendel, pp. 25, 26, on variability of cultivated plants. 



3 Natural Inheritance, p. 18. 



