606 THE PHYSIOLOGY OF REPRODUCTION 



rather to diminish, though the last result may have been due to 

 slight environmental changes. It is possible that fertility, like 

 other characteristics, cannot be increased indefinitely by selective 

 breeding, but that when once the limit existing in the strain 

 has been reached, artificial selection is powerless to effect an 

 improvement. 



CERTAIN CAUSES OP STERILITY 



A detailed account of the various pathological conditions 

 which are capable of inducing sterility is outside the scope of this 

 work. The medical publications dealing with the subject form 

 a very considerable literature, 1 while the causes of sterility in 

 animals are discussed, though somewhat unsatisfactorily, in 

 many of the veterinary text-books. It may not be out of 

 place, however, to refer briefly to certain of the conditions 

 which are known to induce sterility in Man and also in 

 animals. 



In the case of the male an incapacity to procreate is due 

 either to impotence (i.e. inability to perform the sexual act), 

 or to sterility (using the term in the more restricted sense, 

 implying an absence of spermatozoa). Impotence may result 

 from (1) absence of sexual desire, (2) absence of the power of 

 erection and consequent intromission, (3) absence of the power 

 of ejaculating the seminal fluid into the vagina, and (4) absence 

 of the ability to experience pleasure during the act of coition, 

 and at the time of the emission of the semen. 2 Or, according 

 to another classification, the causation of impotence may -be 

 either anatomical, physiological, pathological, or psychological. 

 Among the anatomical causes may be mentioned defects and 

 deformities in the penis. The physiological and pathological 

 causes include incomplete erections, premature ejaculations, 

 diseases of the brain and spinal cord (and more particularly of 

 the centres for the performance of the sexual act), besides such 

 diseases as albuminuria or prolonged diabetes. The psycho- 

 logical causes include fear, repugnance, want of confidence, &c. 3 



1 Miiller (P.), Die Unfruchtbarkeit der Ehe, Stuttgart, 1885. This work 

 contains a bibliography. 



2 Hammond, Sexual Impotence in the Male, New York, 1883. 



3 Corner, Diseases of the Male Generative Organs, London, 1907. 



