356 Professor J. Arthur Thomson [March 30, 



commonplaceness which enables the son of a degenerate father to 

 escape the whole burden of the parental ill." 



At this point one should discuss reversion or atavism, but it is 

 exceedingly difficult to get a firm basis of fact. I use the term re- 

 version to include cases where through inheritance there re-appears 

 in an individual some character which was not expressed in his 

 parents, but which did occur in an ancestor. I say advisedly " through 

 inheritance," in order to exclude those cases where the re-appearance 

 of the character can be accounted for in some other way. The term 

 thus defined is a very wide one, and not very fortunate, but it is 

 difficult to get rid of. I use it to refer to abnormal as well as normal 

 characters, even to include the re-appearance of characters, the normal 

 occurrence of which is outside of the limits of the race altogether, 

 i.e. in some phyletically older race. In other words, the character 

 whose re-appearance is called a reversion may be found within the 

 verifiable family, within the breed, within the species, or even iu a 

 presumed ancestral species.* 



The best illustrations of reversion are furnished by hybrids. 

 Thus in one of Prof. Cossar Ewart's experiments a pure white 

 fan tail cock pigeon, of old-established breed, which in colour had 

 proved itself prepotent over a blue pouter, was mated with a cross 

 previously made between an owl and an archangel, which was far 

 more of an owl than an archangel. The result was a couple of 

 fantail-owl-arch angel crosses, one resembling the Shetland rock- 

 pigeon, and the other the blue rock of India. Not only in colour, 

 but in shape, attitude and movements there was an almost complete 

 reversion to the form which is believed to be ancestral to all the 

 domestic pigeons. The only marked difference was a slight arching 

 of the tail. Similar results were got with fowls and rabbits. 



But great carefulness is necessary in arguing from the results of 

 hybridisation, to those of ordinary mating, and even if some of the 

 phenomena of exclusive inheritance seem to show reversion to a near 

 ancestor we need a broader basis of fact than we have at present 

 before we can formulate any law. It is impossible to read the 

 recorded cases without becoming convinced that many phenomena 

 are labelled reversions on the flimsiest evidence. Thus the occur- 

 rence of a Cyclopean human monster with a median eye has been 

 called a reversion to the sea-squirt, and gout has been called a 

 reversion to the reptilian condition of liver and kidneys. Often 



* Professor Karl Pearson defines a reversion as " the full reappearance in an 

 individual of a character which is recorded to have occurred in a definite ancestor 

 of the same race," and atavism as " a return of an individual to a character not 

 typical of the race at all, but found in allied races supposed to be related, to the 

 evolutionary ancestry of the given race." " In reversion we are considering a 

 variation, normal or abnormal, from the standpoint of heredity in the individual; 

 in atavism we are considering an abnormal variation from the standpoint of the 

 ancestry of the race." But as the two words seem to be used by some authors in 

 the converse way, or as equivalent, and as it is surely difficult to define the field 

 of abnormal variation, I have adhered to the wider usage. 



