ON VAIUETIES, UACES, SUB^SPEClES, A\D srEClEa. T9 



sible modification?, otlicr tilings remaining the same, still observa- 

 tion shows that plants, which are associated in the natural system, 

 are susceptible in very different degrees of modifications : tliis is 

 apparent from the permanence of rye, and the numerous variations 

 which wheat undergoes in different climates and under different 

 modes of treatment. The cause of this difference of susceptibility 

 is still unknown, and is one of the finest problems yet remaining 

 to be solved. 



However it may be, the modifications undergone by so many 

 individuals of difl^erent species being as notorious as the principle 

 of the unchangeableness of their essential characters under the 

 circumstances in which they now live, we think it may be as well 

 to give a comparison that will clearly show our own ideas on the 

 double tendency of individuals in general to preserve their essen- 

 tial characters, and of the possibility of their undergoing some 

 modification. 



If a cylinder of wood or any other homogeneous matter be 

 placed on one of its bases on a horizontal plane, the axis of the 

 cylinder will be pt-rpendicular to the plane, and the equilibrium 

 will be as stable as possible ; but if a force acting in a plane 

 perpendicular to the cylinder and in the direction of its axis 

 moves it from the vertical position, but not sufticiently to upset 

 it, a nev/ equilibrium will be produced which will continue as 

 long as the force is applied. The cylinder may be made to take 

 every possible position with respect to the horizon, and may be 

 made to describe an indefinite number of cones, the largest and 

 external cone being described by the cylinder when it is as 

 much as possible out of the vertical without tumbling over, and 

 the smallest and innermost cone being that described by the 

 cylinder when inclined in the least possible degree from the per" 

 pendicular. 



We will now endeavour to explain the distinctions we have 

 established with respect to the species of living bodies by means 

 of the preceding illustration. 



1. The normal and most stable position of the cylinder is 

 when it is upright on a horizontal plane, and it then corresponds 

 to the ideal case of a species represented by individuals resem* 

 bling each other in every respect. 



2. When the positions in v/hich the angles made by the cy- 

 linder's axis with the vertical are very acute, in consequence of 

 the disturbing force acting but feebly, they then correspond 

 to the differences more or less apparent which distinguish from 

 one another; 1. the individuals of the species Alpha; 2. thn 

 individual types of the species I^eta and Gamma. 



3. 'J'he positions in whicli tlie cylinder's axis makes larger 

 angles with the vertical than the last positions, correspond to 



