358 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 5 



keep alive defective types of many kinds that would be eliminated in 

 wild species through comi^etition. Medicine has been, in fact, largely 

 instrumental in devising means for the preservation of weak types of 

 individuals, and in the near future medical men will, I suggest, often 

 be asked for advice as to how to get rid of this increasing load of 

 defectives. Possibly the doctor may then want to call in his genetic 

 friends for consultation. The point I want to make clear is that 

 the complexity of the genie composition of man makes it somewhat 

 hazardous to apply only the simpler rules of Mendelian inheritance; 

 for the development of many inherited characters depends both on 

 the presence of modifying factors and on the external environment 

 for their expression. 



I have already pointed out that the gene generally produces more 

 than one visible effect on the individual, and that there may be also 

 many invisible effects of the same gene. In cases where a condition 

 of susceptibility to certain diseases is present, it may be that a 

 careful scrutiny will detect some minor visible effects produced by 

 the same gene. As yet our knowledge on this score is inadequate, 

 but it is a promising field for further medical investigation. Even 

 the phenomenon of linkage may some day be helpful in diagnosis. 

 It is true there are known as yet in man no certain cases of linkage, 

 but there can be little doubt that there will in time be discovered 

 hundreds of linkages, and some of these, we may anticipate, will tie 

 together visible and invisible hereditary characteristics. I am aware, 

 of course, of the ancient attempts to identify certain gross physical 

 human types — the bilious, the lymphatic, the nervous, and the san- 

 guine dispositions and of more modern attempts to classify human 

 beings into the cerebral, respiratory, digestive, and muscular, or more 

 briefl}^, into asthenics and pycnics. Some of these types are sup- 

 posed to be more susceptible to certain ailments or diseases than are 

 other types, which in turn have their own constitutional character- 

 istics. These well-intended efforts are, however, so far in advance 

 of our genetic information that the geneticist may be excused if he 

 refuses to discuss them seriously. 



In medical practice the physician is often called upon for advice as 

 to the suitability of certain marriages where a hereditary taint is 

 present in the ancestry. He is often called upon to decide as to the 

 risk of transmitting certain abnormalities that have appeared in the 

 first-born child. Here genetics will, I think, be increasingly helpful 

 in making known the risk incurred and in distinguishing between 

 environmental and hereditary traits. 



Again, a knowledge of the laws of transmission of hereditary 

 characters may sometimes give information that may be helpful in 

 the diagnosis of certain diseases in their incipient stages. If, for 



