186 LIFE ON THE EARTH. 



dification of the form, colour, size, structure, or other 

 particulars ; but the mutations thus superinduced are 

 governed by constant laws, and the capability of so 

 varying forms part of the specific character. 



' 3. Some acquired peculiarities of form, struc- 

 ture, and instinct are transmissible to the offspring; 

 but these consist of such qualities and attributes 

 only as are intimately related to the natural wants 

 and propensities of the species. 



' 4. The entire variation from the original type, 

 which any given kind of change can produce, may 

 usually be effected in a brief period of time, after 

 which no farther deviation can be attained by con- 

 tinuing to alter the circumstances, though ever so 

 gradually; indefinite divergence, either in the way 

 of improvement or deterioration, being prevented, 

 and the least possible excess beyond the definite 

 limits being fatal to the existence of the individual. 



'5. The intermixture of the distinct species is 

 guarded against by the aversion of the individuals 

 composing them to sexual union, or by the sterility 

 of the mule offspring. It does not appear that true 

 hybrid races have ever been perpetuated for several 

 generations, even by the assistance of man ; for the 

 cases usually cited relate to the crossing of mules 

 with individuals of pure species, and not to the 

 intermixture of hybrid with hybrid. 



