312 O. F. COOK 



explanations of Mendel's laws. White mice are now an inbred 

 domesticated variety while the gray mice with which they are 

 compared have had much more recent opportunities of inter- 

 breeding. Recessive gray mice can doubtless be secured by 

 inbreeding, and dominant white mice by interbreeding. 



That mutations like those which " Mendelize " as "pure 

 recessives " should be able to " revert" after man}^ generations, 

 to a parental type by crossing with each other, would also seem 

 to show that the whole question is one of ancestry and methods 

 of descent, rather than of pure germ cells, chromosomes, or 

 character units. Such explanations of Mendelism can only 

 show in higher relief the abnormality of the phenomenon, instead 

 of justifying themselves as general " principles of heredity." 



In the higher plants and animals the conjugation of the par- 

 ental nuclear elements is not completed until the fusion of chro- 

 matin, or mitapsis, has occurred, before the so-called " reduc- 

 ing-division " which precedes the formation of the germ-cells 

 for the next generation. Inability to form normal germ-cells 

 may explain why the line of descent is broken at the stage of 

 sexual reproduction, in sterile mutations and hybrids, though 

 in other cases equally fatal derangements may appear, either 

 before or after the reproductive period. The failure of the 

 chromosomes of sterile hybrids to behave normally is no proof 

 of the existence of a predetermining " hereditary mechanism " ; 

 it is but one of the many related phenomena which show that 

 the evolutionary mischances of hybrids and mutations are not 

 confined to the external form, but may affect an}' part of the 

 organism, and even the cells of which the body is composed. 



MUTATION AND REVERSION. 



Evolutionary debility and derangement through inbreeding 

 are old and well-known facts, but, notwithstanding the frequent 

 use of the term, it has yet to be shown that there is any such 

 phenomenon in nature as reversion, in the strict sense — any 

 actual doubling back upon the evolutionary road. There is 

 sometimes an arrest of development ; accidents or unfavorable 

 circumstances may keep a plant or animal from attaining the 

 normal stature or form of its species, and thus leave it with a 



