272 COOK 



DIFFERENCES OF ABERRANT HEREDITY (TERATISM). 



There are many biological accidents, so to speak, as when in 

 the laboratory, or perhaps in the surf of the sea beach, an egg 

 of one of the simpler animals is shaken apart and develops into 

 two organisms instead of one. In a similar manner, through 

 some mistake of division, two-headed monsters and other mal- 

 formations occur. No less abnormal are many of the freaks 

 which can be produced by unfavorable conditions of growth. 

 Another series of abnormalities is caused by violations of the 

 law of symbasis, that is, through inbreeding which eliminates 

 heterism and normal diversity of descent. 



Teratic characters which are the result of accidents of growth 

 or environment are not inherited, except as they may give rise 

 to a general weakness or debility of the organism. Teratic 

 neisms, on the other hand, are readily heritable. 



Teratisms, like accommodational variations, have received 

 much study, especially from those who hoped to gain from 

 organic derangements an insight into the nature of the agencies 

 by which organic structures are built. The field of teratology 

 affords many interesting and significant data, but the correct 

 interpretation of them has been hindered, as in other departments 

 of evolution, by the confusion of issues which are essentially 

 distinct. There are at least as many kinds of teratisms as there 

 are of normal differences, and probably more, and endless 

 gradations of each kind. This is well illustrated by the phe- 

 nomena of mutation which have received so large an amount of 

 study in recent years. Mutations show all degrees of abnor- 

 mality, and they grade imperceptibly into the differences of 

 normal individual diversity (heterism) as well as into those of 

 normal and prepotent new characters (neism). 



ABNORMAL MUTATIVE DIVERSITY. 



That species are not normally constant and stationary in their 

 characters could not be better proved experimentally than by 

 the many attempts of breeders of plants and animals to maintain 

 constancy of characters in domesticated varieties. Selection 

 conduces at first to such a constancy or uniformity among all 



