49<^ Orga?Jis?ns Are Products of 



always agree, but they often use re- 

 productive ability as a test. If two 

 organisms which are seemingly different 

 can be mated with one another and pro- 

 duce offspring that can live and repro- 

 duce and cross with the parent organism, 

 these two organisms can then be classi- 

 fied as belonging to two varieties within 

 the same species. If they do not readily 

 interbreed they are different enough to 

 belong to separate species. There are 

 many exceptions, however. 



Selection has improved animal types. 

 Many new and better varieties or breeds 

 of animals have been produced by se- 

 lection. For centuries men selected the 

 fastest horses in every generation for 

 breeding. In fact, selection has been 

 practiced in horse breeding so success- 

 fully and for so long a time that in recent 

 years there has been little or no improve- 

 ment in the records set by race horses. 

 Cattle, too, have been improved by se- 

 lection. The best Holstein-Friesian cow 

 in 1880 produced 18,004 pounds of 

 milk during one year. By 1942 this breed 

 had been improved so much that one 

 cow set a new record of 41,943 pounds, 

 and at least one herd of 1 2 cows averaged 

 more than 20,000 pounds of milk per 

 year. This increase was accomplished 

 largely by the careful selection of the 

 best genetic combination in each genera- 

 tion and the use of these animals for 

 breeding. See Exercise 4. 



Improvement of animals is usually 

 slower than improvement of plants be- 

 cause animals can never be self-fertilized. 

 The closest the breeder can get to the 

 self-fertilization found in some plants is 

 to mate close relatives, such as father and 

 daughter or brother and sister. The ge- 



Heredity and Enviromiient unit ix 



netic make-up of close relatives is some- 

 times very similar. Therefore inbreeding, 

 the breeding of close relatives, is prac 

 ticed after successful selection of the 

 most desirable gene combination has 

 been made. In this way new and better 

 breeds of horses, cattle, sheep, and other 

 animals are maintained. 



Inbreeding and outbreeding. It is often 

 said that inbreeding is not desirable, that 

 it leads to offspring that are defective in 

 one way or another. This may happen 

 for the following reason: all organisms 

 have recessive genes and close relatives 

 are likely to have the same recessive 

 genes. In fertilization, two like recessive 

 genes may come together, producing a 

 character that was possessed by neither 

 of the parents. If the recessive gene is 

 undesirable, and many recessives are un- 

 desirable, the inbreeding results in poor 

 offspring. If on the other hand, the par- 

 ents have many desirable genes, inbreed- 

 ing is advantageous; it prevents the intro- 

 duction of less desirable genes. And if 

 undesirable offspring appear through in- 

 breeding thev are discarded, so that in 

 each generation the breed becomes pure 

 for more and more desirable genes. How- 

 ever, except in self -fertilized plants in- 

 breeding has never gone on long enough 

 to produce organisms that are pure in all 

 characters. 



In outbreeding or crossbreeding, two 

 organisms are crossed that belong to the 

 same species or even the same variety but 

 have no family connection. Could you 

 demonstrate the method of outbreeding 

 in plants.^ See Exercise 5. After out- 

 breeding, the offspring sometimes has 

 hybrid vigor. See Figure 444. You will 

 find an explanation for hybrid vigor in 



