488 PHILOSOPHIC ZOOLOGY 



view to producing offspring presenting the special character or 

 characters in an increased degree. 



Objections to the Theory of Natural Selectiox. — At 

 various times a number of objections have been made to the 

 theory, some being of a very trifling and quibbhng sort, others 

 of more serious nature. It is no part of the plan of this book 

 to enter into all the difficulties that require or have required to 

 be met, and it may suffice to mention one of the most formidable 

 objections, derived from the supposed " swamping effects of inter- 

 crossing ". That is to say, supposing a favourable variation to 

 have arisen, it seems at first sight that it is just as likely to 

 gradually disappear again by intercrossing as to be emphasized 

 by heredity, indeed more likely. Fleeming Jenkin, the first 

 propounder of this difficulty, illustrated it by the possible case 

 of a white man becoming king of a black island population, his 

 whiteness typifying a favourable variation. His immediate de- 

 scendants would not be white, but yellow, and in the course of 

 several generations the royal house would probably be just as 

 black as their subjects. 



It may be added that if a number of domesticated races of, 

 say, pigeons, are allowed to cross freely together, their peculiar 

 characteristics gradually disappear, and the features possessed 

 by the original wild stock are reacquired. This is generally 

 explained as a case of reversion or atavism, or in more popular 

 language, a "throw back" to the ancestral type. 



The objection has been often met by supposing that the 

 particular variation occurred not in one, but in a number of 

 individuals, thus giving a better start for the formation of a new 

 species, but such an idea requires proof. Even if we admit the 

 probability of the occurrence, the factor of Isolation must be 

 emphasized, as has been done by Romanes and others. Isola- 

 tion of individuals presenting a certain kind of variation would 

 certainly prevent the swamping effect of intercrossing from oper- 

 ating, and pigeon-fanciers, for example, could never succeed in 

 producing new breeds if they did not sort out and keep their 

 birds separate, according to their special requirements. Such 

 isolation actually occurs in nature when a small number of indi- 

 viduals belonging to a particular species reach, .say, an oceanic 

 island, where adaptations to a new set of surroundings become 

 necessary, and where they are separated from the original home 



