236 LAY SERMONS, ESSAYS, AND REVIEWS. [xn. 



that &quot;no other characters than those founded on the develop 

 ment of bone for the attachment of muscles&quot; are capable of 

 variation. In precise contradiction of this hasty assertion, 

 Mr. Darwin s researches prove that the skeleton of the wings in 

 domestic pigeons has hardly varied at all from that of the wild 

 type ; while, on the other hand, it is in exactly those respects, 

 such as the relative length of the beak and skull, the number of 

 the vertebrae, and the number of the tail-feathers, in which 

 muscular exertion can have no important influence, that the 

 utmost amount of variation has taken place. 



We have said that the following out of the properties ex 

 hibited by physiological species would lead us into difficulties, 

 and at this point they begin to be obvious ; for if, as the result 

 of spontaneous variation and of selective breeding, the progeny 

 of a common stock may become separated into groups dis 

 tinguished from one another by constant, not sexual, morpho 

 logical characters, it is clear that the physiological definition of 

 species is likely to clash with the morphological definition. No 

 one would hesitate to describe the pouter and the tumbler as 

 distinct species, if they were found fossil, or if their skins and 

 skeletons were imported, as those of exotic wild birds commonly 

 are and without doubt, if considered alone, they are good and 

 distinct morphological species. On the other hand, they are not 

 physiological species, for they are descended from a common 

 stock, the rock-pigeon. 



Under these circumstances, as it is admitted on all sides that 

 races occur in Nature, how are we to know whether any ap 

 parently distinct animals are really of different physiological 

 species, or not, seeing that the amount of morphological dif 

 ference is no safe guide ? Is there any test of a physiological 

 species ? The usual answer of physiologists is in the affirmative. 

 It is said that such a test is to be found in the phenomena of 

 hybridization in the results of crossing races, as compared with 

 the results of crossing species. 



So far as the evidence goes at present, individuals, of what 



