CHAPTER IV. 



The Advantage of Changed Conditions of Life upon the Vital Organism. 



It is a well-established fact that, within certain limits, changed 

 conditions of life are beneficial to both the plant and animal 

 organism. This fact has long been known to horticulturists 

 and breeders of animals. In order to obtain the best results, 

 the horticulturist takes care to vary continually the soil of 

 the successive generations of a plant, and breeders of animals 

 are equally careful not to confine several successive generations 

 of the same breed to a narrow and fixed locality.* 



How are we to explain the beneficial effects thus obtained ? 



According to Herbert Spencer, the forces of Nature are 

 ever working towards an equilibrium. Thus, when a fire 

 burns, the tendency is towards the arrangement of atoms in 

 their most stable form. In like manner the aggregated forces 

 constituting a living organism tend to fall into a state of stable 

 equilibrium. Such a condition is, however, inconsistent with 

 vital activity, in which the acting forces, far from being stable, 

 are undergoing incessant change. By change of E, a new 

 series of force is introduced, so that the approach towards 

 equilibrium is for the time checked, and this has a beneficial 

 effect upon the body processes. 



Darwin explains the beneficial effects of changed conditions 

 of life in a very similar way, and he believes that the neces- 

 sity for sexual reproduction admits of a like inteq^retation. 

 We have seen that the whole purpose of sexual reproduction 

 is to secure the union of two unlike individuals. Such a 

 union, according to Darwin, tends, by a disturbance of forces, 

 to check the progress towards equilibrium, in the same manner 

 as changed conditions of life ; for when the germ and sperm 



* See on this subject Darwin, "Variation under Domestication," Vol. II. 

 Chap, xviii. 



