352 "Professor J. Arthur Thomson [March 30, 



III. Different Degrees of Hereditary Resemblance. 



Before the middle of the century considerable attention was paid 

 to what might be called the demonstration of the general fact of 

 inheritance. In a big treatise like that of Prosper Lucas (1847) 

 many hundreds of pages are devoted to proving what we now take for 

 granted — that the present is the child of the past, that our start in 

 life is no haphazard affair, but is rigorously determined by our 

 parents and ancestors, that various peculiarities, normal and ab- 

 normal, physical and mental, may re-appear generation after genera- 

 tion, and so on. One step of progress during the Darwinian era has 

 been the recognition of inheritance as a fact of life which requires no 

 further proof. 



Yet this aspect of the study of heredity is by no means worked 

 out. Thus there are some characters, e.g. tendencies to certain 

 diseased conditions, which are more frequently transmitted than 

 others, and we ought to have, in each case, precise statistics as to the 

 probabilities of transmission. 



Again, there are some subtle qualities whose heritability must not 

 be assumed without evidence. Thus it is of very great importance 

 to students of organic evolution that Prof. Karl Pearson has recently 

 supplied, for certain cases, definite proof of the inheritance of fecun- 

 dity, fertility and longevity. 



The familiar saying, " like begets like," should rather read, " like 

 tends to beget like," since variation is quite as important a fact as 

 complete hereditary resemblance. If it seems to us that in many 

 cases the offspring is practically a facsimile reproduction of the 

 parent, this may be due to absence of variation, or, what comes 

 almost to the same thing, to great completeness of inheritance ; but 

 it is more likely to be due to our ignorance, to our inability to detect 

 the idiosyncrasies. 



But it will be granted by all that the completeness with which 

 the characters of race, genus, species and stock are reproduced gene- 

 ration after generation, is one of the large facts of inheritance. It is 

 obvious, however, that this does not sum up our experience, and we 

 must face the task of considering what may be called the different 

 degrees of hereditary resemblance. For these a confused classification 

 and a troublesome terminology have been suggested, to discuss which 

 would be most unprofitable in the limits of a short lecture. 



I therefore propose to restrict attention to three familiar cases, 

 which are called blended, exclusive, and particulate inheritance, and 

 then to say a few words in regard to the phenomena known as 

 regression, reversion and atavism. 



A preliminary consideration must be attended to. It is a matter 

 of observation that there are great differences in the degree in which 

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

 ture that lack of resemblance is necessarily due to incompleteness 



