.8,,. BIOLOGICAL THEORIES. 585 



"atavistic" monstrosities as being of the same nature as the true 

 atavism, shown for instance in the " reversion " of domestic breeds of 

 pigeons to the type of the blue rock pigeon. 



Monstrosities Hke two-toed horses, human beings with " per- 

 sistent gill-clefts," and the like, belong to a class of pathological 

 phenomena which are, in my opinion, in no way connected, or at most 

 are only remotely connected with true atavism. 



That the blending of two constitutions by hybridism should 

 lead to a partial blending of characters distinctive of those two con- 

 stitutions, is what might perhaps be expected a priori; and the 

 exaltation of characters possessed in common by the two constitutions 

 is similarly to be expected, but it is to be remembered that it is stock- 

 constitutions, not individual-constitutions, to which I refer. In the case 

 of pigeons, the characters common to all breeds are those implied by the 

 name "pigeon." All pigeons alike have certain anatomical characters, 

 but there are other characters which distinguish each breed from the 

 others. There are certain constitutional characters common to all 

 domestic breeds ; for instance, all appear to be liable to " atavism," 

 though its occurrence is comparatively rare. Experiment has shown 

 (Darwin) that hybrids do possess some of the characters of the sepa- 

 rate breeds from which they are derived, and that they are more liable 

 to atavism than pure-bred birds. 



Before proceeding to consider some more complex cases of 

 helredity, it is well to summarise the conclusions reached so far. They 

 are the following : — 



(i) Heredity is essentially a limitation of variation, and (2) is therefore of 

 less antiquity than variation. (3) It is not an absolute limitation ; variation still 

 occurs. (4) The limitation has been effected by " natural selection " {using that 

 term in a widened sense) operating upon the stock-constitution. (5) The limitation 

 has been progressive, the range of variation having been narrowed step by step ; 

 the frequency of variation having been diminished step by step ; and the number 

 of characters whose variation has been thus affected having been increased step by 

 step. (6) The efficient cause of that limitation, that is, " natural selection,'' being 

 removed, the range and the frequency of variation increase. (7) A new selection 

 leads to a new set of " hereditary characters,'' and this process is similar to that 

 by which ^^ heredity " has arisen. (8) The phenomenon of true atavism is simply 

 the occurrence of those characters in a given individual which selection, natural or 

 artificial, rendered less and less frequent, but the frequency of which has not yet 

 been reduced to zero, (g) The blending of two stock-constitutions does in fact 

 lead to the phenomena to be expected ; notably to a combination of various 

 features characteristic of the two stocks, and an exaltation of at least one charac- 

 teristic common to the two constitutions, i.e., the occasional production of atavistic 

 offspring. 



So far only the more ordinary types of heredity have been con- 

 sidered, those types *and varieties which other theorists have sought 

 to explain, that is, those cases in which the production of individuals 

 fitted for successful competition in the struggle for existence, has been 



