210 THE ZOOLOGIST. 



and unfavourable conditions would ultimate!} 7 lead to the de- 

 struction of the species, instead of these being able to adapt 

 themselves to the altered conditions. The power of adaptation 

 to new conditions in a species is therefore simply another ex- 

 pression of the law of variation. 



The most striking proof of the existence of a disposition 

 towards variation is the occasional occurrence of a sudden varia- 

 tion, which is known as a " sport" when appearing in plants, and 

 as a " monstrosity " when among animals. Besides such abrupt 

 and very striking manifestations of this law, a perpetual varia- 

 bility productive of lesser differences pervades all animated 

 nature. This becomes evident when we take into account the 

 minute differences which appear between the individuals of every 

 existing species. It is a well-known fact that not a single 

 animal or plant can be found which is identical with any other 

 of its kind, not even among those that emanated from the same 

 parent at one birth, nor is there a single leaf of the same tree, or 

 the many thousands of them that may be found in the forest, 

 which does not possess some minute peculiarity of its own found 

 in no other of its kind. Through that most powerful and uni- 

 versal law of nature, the law of inheritance, the general character 

 of variation, if beneficial to the individual, may be propagated 

 and increased under natural conditions, and still more so by 

 artificial means ; often, indeed, under unnatural conditions. In 

 fact, the law of variation is extensively made use of by man, and 

 worked upon by him through selection, which, under domesti- 

 cation, is frequently carried in a direction that would be disastrous 

 to the existence, or at least very disadvantageous, to the species 

 under natural conditions. 



The pig, for example, is bred for the production of flesh and 

 small bones. Through domestication it has lost its abundant 

 covering of hair, and to a large extent its strength of jaws and 

 the size of its tusks. Under the protection of domestication the 

 pig does not suffer by these degenerations, because it is arti- 

 ficially fed, and, where needed, guarded against the extremes of 

 the climate. With natural surroundings these detrimental varia- 

 tions could never have reached the extent we find them to exist, 

 for the obvious reasons that the want of a strong covering of hair 

 would expose the animal to the extremities of the climate, the 

 absence of a strong jaw would hinder it getting sufficient food by 



