OLD TYPES AND NEW 151 



monotreme, are all mammals which have developed 

 in a similar manner the very peculiar device of der- 

 mal spines. There is no reason, however, for regard- 

 ing this character as due to descent from a common 

 spiny ancestor. It is not reversion to an ancestral 

 type, but rather a case of convergent variation. 

 Similarity does not always indicate genetic continuity. 

 In the case of birds albinism, mefamsm and fla^ 

 vism are modifications of ordinary pigmentation which 

 appear irregularly among many different species as 

 pathological " sports," but no one of these conditions 

 can be regarded as reversions to ancestral white, 

 black, or yellow types. 



e. Regression 



Galton's "law of regression" refers to the wide- 

 spread phenomenon already explained of a constant 

 swinging back to mediocrity which the breeder must 

 oppose with continual selection in order to maintain 

 the standard of any particular strain. We have 

 seen that within a "pure line," regression is complete 

 and that in populations made up of a mixture of 

 pure lines it is a factor always to be reckoned with. 

 Regression, however, has to do with fluctuating varia- 

 tions and does not bring about a permanent change 

 of type. It should, therefore, not be confused with 

 reversion. 



3. EXPLANATION OF REVERSION 



Darwin, who did not always differentiate between 

 reversion and atavism, suggested that reversion was 



