MODERN THEORIES OF HEREDITY'. 



I'.v Walter T. Saxton, M.A., F.L.S. 



in attempting to set forth within the limits of a single paper 

 of limited length the ideas which underlie the two main theories, 

 one or other of which is supported by practically all students of 

 heredity to-day. 1 feel that the task is no light one. The subject 

 is complicated by the obvious necessity of reviewing as briefly as 

 possible the main theories which have preceded those which now- 

 bold the field. 



The first of these which it is necessary for us to consider is 

 that associated with the name of Lamarck, and which was after- 

 wards strenuously supported by Herbert .Spencer. According 

 to Lamarck's view, the slight fluctuations which all organisms 

 exhibit (as instanced by the fact that no two men are exactly 

 alike in every respect ) are quite unimportant in heredity, but 

 those characters acquired by an oiganism in the course of its 

 development (e.g., strength of certain muscles through use) are 

 those which are chiefly inherited by the offspring. I am of 

 opinion that any sensible man, by observation of some of the 

 more obvious facts of everyday life, would come to the same 

 conclusion, and Herbert .Spencer has collected many interesting 

 data which are very difficult to explain on any other hypothesis. 



Nevertheless Darwin, h\ more extended observation, aided 

 by a number of experiments, was led to cjuite a diiTerent theory 



It is perhai)s desirable, at this point, that we should make a 

 clear distinction between the terms " observation " and " experi- 

 ment." By an " observation " I mean the record of an observed, 

 phenomenon ( ustially as it occurs in nature) when one or more 

 of the conditions under which that phenomenon takes place are 

 not under the observer's control, and are usually wholly or partly 

 unknown. An " experiment." on the other hand, signifies the 

 occurrence of a i)henomenon (not. as a rule in nature) under 

 conditions which are controlled by the observer. Li the ideal 

 experiment all the conditions are known and controlled, but often 

 this is impossible. To further emphasize the distinction bv 

 means of an example : — llie way in which green ])lants obtain 

 Cheir organic food, from the carbi.n dioxide of the air. could 

 never be determined by "" observation " alone, but can l)e readilv 

 ascertained by " experiment." 



Darwin's theory of heredity, then, was based i)artlv on 

 observation, and partly on experiment. It was to tlie efi:'ect that 

 minute fluctuations, such as occur in all organisms, are. or tend 

 to be, inherited In- the (offspring, and that " natural selection," in 

 other words the " survival of the fit," will " fix " such variations 

 as are advantageous, while i)lants or animals inheriting a varia- 

 tion in a contrary direction will l)e elinn'nated in the struggle for 

 existence. Darwin ap])ears never to have discussed Lamarck's 



