CHAPTER I. 



INTRODUCTORY.' 



All domestic breeds of pigeons, of which there are probably not less than 200 

 distinct kinds, are fertile when crossed and the offspring of crosses are also fertile. 

 The same holds true, so far as known, between the domestic races and the wild 

 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 are nearly 500, crosses 

 are very often infertile, and fertile hybrids are so rare that Darwin could not find 

 "a single well-ascertained instance of hybrids between two true species of pigeons 

 being fertile inter se, or even when crossed with one of their pure parents" (Animals 

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

 "instance" he vainly sought for. 



Even if fertile hybrids were sometimes obtainable, the possibility of establishing 

 an "intermediate" race between two wild species would presumably be very 

 doubtful, since such a result as Darwin points out is extremely difficult even in 

 the case of domestic breeds where the hybrids are perfectly fertile. Darwin says: 

 "Certainly, a breed intermediate between two very distinct breeds could not be 

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

 record of a permanent race having been thus formed." (Origin of Species, 5th ed., 

 p. 33.) 



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

 very meager; with comparatively few exceptions, domestic animals and cultivated 

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

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

 underestimated. 



The difficulty with domestic forms is that they have a complex ancestry which 

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

 most tangled series of hybridizations,- and hence they give, for the most part, 

 unpredictable results — "reversions," "regressions," "mutations," "sports," et id 

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

 viewed as the fulfillment of only one out of an indefinite number of mutually exclu- 

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

 characters that may sleep for generations and then appear as variations which obey 

 no law except "the law of large numbers," which virtually says: "There is no 

 explanation for the individual phenomenon; be satisfied to know that it is one of 

 many phenomena which always follow that wonderful curve of chance which 

 defined itself by Newton's binomial theorem, and which is so useful in its application 

 to life insurance, pension laws, etc." 



Without demurring to the many interesting applications of "the law of large 

 numbers," it must not be forgotten that the individual phenomenon fulfills itself 



'This paper was written (1904-5) under the caption "Hybrids of Wild Species of Pigeons."— Editor. 

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

 ' Natural Inheritance, p. 18. 



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