60 GENETICS 



cestor of the rodents probably had five toes on each 

 foot, but the normal number in modern guinea-pigs 

 is four on each of the front feet and three on the 

 hind feet. The individual .from which Castle has 

 bred a four-toed race exhibited a degressive muta- 

 tion, tending toward the ancestral type. 



5. SPECIES AND VARIETIES 



The doctors have always disagreed regarding a 

 definition of species. What determines the exclusive 

 boundaries that shall isolate from their fellows any 

 particular group of animals or plants has long been 

 a mooted question, and still remains so. 



The Linnaean concept of a species was that of an 

 exclusive caste of individuals, inflexibly demarked, 

 over whose high barriers no nondescript tramps 

 would dare attempt to climb. When an entomolo- 

 gist of the old Linnsean school encountered an insect 

 which did not conform to the morphological tradi- 

 tions of its fellows, the frequent fate of such a non- 

 conformist was to perish under the boot-heel rather 

 than to find sanctuary in the cabinet of the pre- 

 served. Since it was an exception, and a violator 

 of the divine law of the fixity of species, it deserved 

 to be annihilated! Those were hard days both for 

 heretics and for mutations. 



The method of the older school of systematists 

 may be described as one which emphasized differences 

 and put up barriers that should keep the unlike 

 apart, at the same time allowing only "birds of a 

 feather to flock together." It was a brave and sue- 



