HISTORY OF ZOOLOGY 35 



Artificial Selection. — In the development of this principle Darwin 

 started with the limited and hence easily comprehended subject, the arti- 

 fical breeding of domesticated animals. Whether these be the descendants 

 of a single species or have arisen from crosses of two or more species 

 (authorities are not in agreement in all cases) they behave like repre- 

 sentatives of a single species. How have the various races and sub- 

 race of pigeons, horses, cattle, dogs, etc. arisen? Darwin finds the 

 causes of these great differences in artificial selection, practised by man for 

 thousands of years. The method is to choose from the stock individuals 

 showing the tendency toward the desired ideal in even the slightest degree 

 more than their fellows, and then pairing these. By repetitions of this 

 selection and breeding, the desired goal is slowly reached. 



This artificial selection depends upon three factors: (i) Variability; 

 the descendants of one pair of parents have the capability of developing 

 new characteristics, thereby differing from their parents. (2) Hered- 

 itability of newly-acquired characters; the tendency of the daughter- 

 generation to transmit the newly-developed characteristic to the 

 succeeding generation. (3) Artificial selection; man selects for breeding 

 suitable individuals, and prevents a new character which has arisen 

 through variation from disappearing by crossing with animals of the 

 opposite variational tendencies. 



If we compare with the facts of domestication the conditions of animals 

 living in the state of nature, we find again variability and heredity, as effici- 

 ent forces, inherent in all organisms, though the former is not everywhere 

 of the same intensity. There are many species w^hich vary only slightly 

 or not at all, and therefore have remained unchanged for thousands of 

 years. But contrasted with these conservative species are in every group 

 plastic species, which are in the process of rapid change, and these alone 

 are of importance in causing the appearance of new forms. Since 

 heredity is present in all organisms, there is only lacking a factor corre- 

 sponding to artificial selection, and this Darwin discovered in the so-called 

 'natural selection.' 



Natural Selection : Struggle for Existence. — Natural 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 parasites, 

 whose eggs are numbered by millions. For the development of this 

 multitude of germs there is no room on the earth. In order to preserve 

 the balance of nature great numbers of unfertilized and fertilized eggs, 

 as well as young animals and many that are mature but have not yet 

 attained their physiological destiny, must perish. Many individuals wiU 



