XV.] THE LAAVS OF HABIT. 181 



appearance of habits is sometimes the result of favour- 

 ing circumstances, sometimes spontaneous. Eeversion to Reversion. 

 ancestral characters is a case of the reappearance of habits. 



AVhen a stimulus is responded to, it strengthens in force impi-es- 

 with repetition ; when it is not responded to, it weakens. ®i°"^- 



Organs strengthen and enlarge with exercise ; and, con- ^g^^^^* ° 



on 



versely, they weaken and diminish with disuse. organs. 



It is now time to consider, in more detail than I have 

 yet done, the maimer in Avhich the characters of a race 

 will be modified by changes in the circumstances under 

 which it has to live. 



It is a universal law, that the health, and ultimately the Great 

 life, of any organism whatever will be destroyed by any are de- 

 very great change in external circumstances. The most struchve. 

 obvious instances of this law are the familiar facts, that 

 air-breathing animals will die in the water, and water- 

 breathing ones will die in the air. These facts, however, 

 do not throw much light on any law of life, for they admit 

 of a purely physical explanation. It is physically impos- 

 sible, quite irrespective of any law of life, that a man's 

 lungs should breathe water, or that a fish's gills should 

 breathe air. But, independently of physical reasons like 

 this, all great changes are destructive of health and life. 

 Cold regions and warm ones, moist places and dry ones, 

 have all their own peculiar races of animal and vegetable 

 inhabitants ; and those species which are native to one 

 kind of abode will, as a general rule, be destroyed by 

 transplanting to a totally different one. Were it not so, 

 differences of climate would be no barrier to the migrations 

 of species, instead of being, as they often are, the most 

 impassable of all barriers. In many cases we cannot say 

 what is the reason of this inability of organisms to adapt 

 themselves to new circumstances. Sometimes, in all pro- 

 bability, it is in part merely physical : for instance, animals 

 with a coat only of hair may be unable to endure the cold 

 of those countries where most of the native quadrupeds 

 are clothed with fur. But tliis kind of reason cannot be 

 given in every case. It is, I think, quite impossible to 



