498 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



such a principle as the inheritance of acquired characters, but even here the 

 principle of natural selection may perhaps be equally explanatory. 



The Inheritance of Diseases. — The question of the inheritance of diseases 

 has also been much discussed. The same general principles apply here as in 

 the inheritance of normal characteristics. The fact has been mentioned above 

 that pathological characters, whether anatomical, physiological, or psycholog- 

 ical, are capable of transmission. If, however, a pathological character has 

 been acquired by the parent and is not inherent in his own germ-cells, it 

 is extremely doubtful whether it can be passed on to the child. A diseased 

 parent, on the other hand, may produce offspring that are constitutionally 

 weak or that are even predisposed toward the parental disease, and such off- 

 spring may develop the parent's ailment. In such cases constitutional weakness 

 or predisposition, and not actual disease, is inherited ; the disease itself later 

 attacks the weak or predisposed body. Proneness to mildness or severity of, 

 and immunity toward, certain diseases seem to be transmissible. These sub- 

 jects, however, are so little understood, and the real meaning of such terms as 

 predisposition, inherited constitutional weakness, and inherited immunity, is so 

 little known, that it is idle to discuss them here. 



Considerable experimental work has been performed recently upon the 

 transmissibility of infectious diseases. Undoubtedly infectious diseases cling 

 to a particular family for generations. The transmitted factor is probably fre- 

 quently, if not usually, simple predisposition. But in an increasing number 

 of cases there appears to be transmission of a specific micro-organism. Such 

 transmission is called germinal when the micro-organism is conveyed in the 

 ovum or the semen, and 'placental or intra-uterine when the micro-organism 

 reaches the fetus a Tier uterine development has begun, and chiefly through the 

 circulation. Of germinal infection- syphilis seems undoubtedly capable of 

 transmission within either the ovum or the semen. The possibility of germinal 

 transmission of tuberculosis has been maintained, but is not fully proven. Of 

 intra-uterine infections there have been observed in human beings apparently 

 undoubted cases of typhoid fever, relapsing fever, scarlatina, endocarditis, 

 small-pox, measles, croupous pneumonia, anthrax, syphilis, and possibly 

 tuberculosis and Asiatic cholera. It is obvious that neither germinal nor 

 placental inheritance, both taking place through the medium of a specific micro- 

 organism, and not through the modification of germ-plasm, is comparable to 

 inheritance in the customary sense. 



Theories of Inheritance. — From early historical times theories of inher- 

 itance have not been wanting. Physical and metaphysical, materialistic and 

 spiritualistic theories have had their day. Previous to the discovery of the 

 spermatozoon (Hamm, Leeuwenhoek, 1677) all theories were necessarily 

 fantastic, and for nearly two hundred years later they were crude. The 

 theories that are now rife may be .-aid to date from 1864, when Herbert 

 Spencer published his Principles <>f Biology. Since that date they have 

 become numerous. Even the modern theories are highly speculative ; none 

 can be regarded as being accepted to the exclusion of all others by a large 



