406 PROGRESS OF SCIENCE IN THE CENTURY. 



pressed. It will be granted by all that the complete- 

 ness with which the characters of race, genus, 

 species, and stock are reproduced generation after 

 generation, is one of the large facts of inheritance. 

 But it is obvious that this does not sum up our ex- 

 perience. The familiar saying, " like begets like," 

 should rather read, " like tends to beget like," for 

 variation is a more frequent occurrence than complete 

 hereditary resemblance. An offspring cannot be a 

 facsimile reproduction of both its parents. If it 

 seem to show no characteristic which its parents did 

 not between them possess, this may be due to absence 

 of variation, or, what comes almost to the same thing, 

 to completeness of inheritance, but it is more likely 

 that the apparent completeness of resemblance is a 

 fallacious inference due to our inability to detect the 

 idiosyncrasies. 



The popular platitude, " the child is a chip of the 

 old block," will not suffice ; there are some characters, 

 e.g., tendencies to certain diseased conditions, which 

 are more frequently transmitted than others, and the 

 student of inheritance must work towards precise 

 statistics of the probabilities of transmission; there 

 are some subtle qualities whose heritability must not 

 be assumed without evidence, thus it is of great im- 

 portance to students of organic evolution that Prof. 

 Karl Pearson has recently supplied, for certain cases, 

 definite proof of the inheritance of fecundity, fer- 

 tility, and longevity. 



Before we notice some of the common modes of 

 inheritance, we must emphasize a preliminary con- 

 sideration. It is a matter of observation that there 

 are great differences in the degree in which offspring 

 resemble their parents ; but it is a matter of conjec- 

 ture that lack of resemblance is necessarily due to 



