INTERNAL CAUSES OF VARIATION 20 1 



SECTION XII PHYSIOLOGICAL SELECTION 



The term " physiological selection " refers to the fact that cer- 

 tain individuals, fertile enough of themselves, will yet absolutely 

 fail to breed with a particular individual of the opposite sex. 1 



This principle is now well established and is recognized as a 

 large cause of fruitless marriages. Individuals are frequently 

 barren in one marriage and perfectly fertile in another. Physio- 

 logical selection is a phase of genetic selection, and while of 

 extreme importance in the marriage relation it constitutes no 

 special menace to our herds. In general it has little bearing 

 upon the development of a breed, but is often exceedingly 

 troublesome when it is desired to effect a particular combination 

 of blood lines. 



SECTION XIII SELECTIVE DEATH RATE; LONGEVITY 



The total population depends as much upon longevity as upon 

 fertility and the prevailing type at any moment depends as much 

 upon the individuals that die out of the world as it does upon 

 those that are brought into it. 



If the draft by death is equal, or rather proportional, from all 

 types of the race or breed, then the existing type will be the 

 same as that born into the world ; if not, it will be different. 



As there is little use in attempting to breed a strain, however 

 desirable, that is not at least fairly prolific, so there is little use 

 in spending time and expense upon short-lived strains, especially 

 of milch cows and horses, which are valuable largely in propor- 

 tion to age. 



For reproductive purposes the " age " of an animal is the age 

 at which he stops breeding, but for other purposes it is the age 

 at which he can no longer render valuable service in the desired 

 direction, such as labor. 



1 This principle was first announced by Romanes (" Physiological Selection," 

 Journal of the Linnccan Society, XIX, 337-41 1), though what he had in mind evi- 

 dently included what is now known as "genetic selection." It was proposed as 

 showing that other principles are at work to fix types, aside from the struggle for 

 existence. 



