THE PRACTICE OF BREEDING 285 



breeders to establish a guide to future practice in the 

 improvement of animals? Answers to these questions 

 not only have great value to practical breeders but have 

 profound biological significance. 



270. Natural selection. The theory of natural selec- 

 tion which has so long influenced and determined the 

 trend of zoological thought is an attempt to explain how 

 the qualities of animals in nature have come to be. 

 Through natural selection organic beings tend to adapt 

 themselves to their surroundings. The more nearly 

 the qualities of animals are favorable to a successful 

 existence under the cqnditions surrounding the individual, 

 the more certainly will the animal live and reproduce. 



The insect that rests upon the branches of trees will 

 escape insectivorous birds more certainly if the color of 

 the body imitates that of the bark upon which it rests. 

 The color of the jungle animal blends so completely into 

 the general landscape that its presence is not detected 

 by its enemies. 



The shore birds or waders in their pursuit of food in the 

 shallow waters of the shore have developed long legs. The 

 giraffe, feeding as it does upon the leaves on the branches 

 of trees, has found a long neck to be a desirable quality in 

 securing food. And in this case the longer the neck the 

 more certainly will the animal survive when food is scarce. 



Darwin assumed that under conditions similar to those 

 mentioned, animals that vary slightly in the desired 

 direction would be preserved and would reproduce, while 

 those animals that varied away from the valuable quality 

 would perish. An animal might continue to develop 

 indefinitely in a given direction through continuous 

 variation. Darwin also recognized the existence of 

 discontinuous variation. 



