HISTORY OF ZOOLOGY. 53 



teristics cumulate ; the number of pigeons having an in- 

 creased number of tail-feathers becomes. greater, and thus 

 material is obtained which is adapted to a further increase 

 in the number of feathers. 



Factors of Evolution in Breeding. The remarkable 

 results of breeding are well known to every observer of our 

 domesticated animals. These results depend mainly upon 

 three factors: (i) [ 'ariability ; the descendants of one pair 

 of parents have the capability of developing new character- 

 istics thereby differing in appearance from their parents. (2) 

 Hcrciiitability of newly-acquired characters. This consists 

 in the tendency of the daughter-generation to transmit the 

 newly-developed characteristic to the succeeding genera- 

 tions. (3) Artificial selection ; man selects for breeding pur- 

 poses suitable individuals, but is hindered by the fact that 

 a new character which has arisen through variation disap- 

 pears through crossing again with animals of the opposite 

 variational tendencies. 



Factors of Evolution in Nature. If we compare with 

 the facts of domestication the conditions of animals living 

 in the state of nature, we find again as efficient forces, in- 

 herent in all organisms, variability and heredity, though 

 the former is not everywhere of the same intensity. There 

 are many species which vary not at all or only slightly, and 

 therefore have remained unchanged for thousands of years. 

 But over against these conservative species stand in every 

 group progressive species, active species, which are in the 

 process of rapid change, and therefore are the only ones of 

 importance in causing the appearance of new species. Since 

 the power of heredity is present in all organisms, there is 

 only lacking a factor corresponding to artificial selection, and 

 this Darwin discovered in the so-called " natural selection." 



Natural Selection : Struggle for Existence. Nat- 

 ural selection finds its basis in the enormous number of 

 descendants which every animal produces. There are 

 animals, e.g. most fishes, which produce many thousands 

 of young in the course of their lives; not to mention para- 

 sites, whose eggs are numbered by millions. For the de- 



