I. 



Biological Theories. 



II.— THE EVOLUTION OF HEREDITY. 



IN the previous article it was contended that the phenomenon 

 called " heredity " was merely a likeness of effects due to 

 likeness of the causes producing them ; the like effects being two 

 similar individuals belonging to different generations of the same 

 species, the like causes being the influences internal and external 

 wliich have determined the form, structure, and constitution of those 

 two individuals respectively. 



The present paper attempts to account for the recurrence, 

 generation after generation, of those " like causes." 



In the search for the efficient cause of this recurrence, that is 

 for the cause of heredity, it is necessary to remember and compare 

 the diverse forms in which heredity is exhibited, and at the same 

 time to take due cognisance of the existence and nature of variation. 



The following facts will serve as types of the groups of 

 phenomena which it is especially necessary to bear continuously in 

 mind with reference to heredity :— 



1. A man not infrequently resembles his father in some features 

 which are comparatively rare in other men. 



2. All Rhahdonemata closely resemble their grandparents, while 

 they differ so widely from their parents {Rhahdites) as to have been 

 referred to a distinct genus. Both Rhabdonema and Rhabditis are 

 produced sexually. 



3. " Soldiers " among one generation of Tennes are very like 

 " soldiers " of previous generations, and yet quite unlike their parents, 

 and unlike any ancestor whatever, however remote. The occurrence 

 of " soldiers " among the offspring of several species of Tennes, 

 which geographically are widely separated, appears to point to their 

 occurrence as a phenomenon of very great antiquity. 



4. In Phylloxera each generation consists of individuals like each 

 other, except in sex, but unlike any individual of the three or four 

 preceding or succeeding generations. In each year there is a cycle 

 of unlike generations, but the individuals of any one generation 

 closely resemble those of the corresponding generations of previous 



