72 THE CONTROL OF LIFE 



that a particular character in the body may be the 

 product of many genes. Perhaps we have said enough 

 to indicate that the Mendelian theory admits of more 

 elasticity than it seemed at first to tolerate — an elas- 

 ticity which the facts of the case appear to demand. 



(e) The mode of inheritance called reversionary or 

 atavistic seems to be of very rare occurrence. It is 

 illustrated when a character known to have marked a 

 distant ancestor remains latent for several generations, 

 and then suddenly finds expression again, like a seed 

 that has been lying dormant for years in an unculti- 

 vated corner of a garden. It turns out, however, that 

 many of the * hark-backs ' which have been called 

 reversions and regarded as due to the reawakening of 

 a factor which has been latent for generations, admit 

 of a much simpler interpretation. They may be con- 

 veniently described as ' reversions,' but they are not 

 really of that nature. For many of them are due to 

 arrested development — to the stoppage of development 

 at too early a stage, perhaps because of some defect 

 in nutrition. Hare-lip in Man seems to be of this nature 

 — ^not a true reversion. 



But many so-called ' reversions ' have another ex- 

 planation, which we owe to the Mendelian school. A 

 strict reversion is due to the reawakening of a char- 

 acter which has lain latent for ages, but what looks 

 like a reversion may be due to a coming-together-again 

 of characters which have been analysed apart in pre- 

 vious generations. When domesticated rabbits of dif- 

 ferent colours are bred together promiscuously their 



