STRUCTURE AND DEVELOPMENTAL RATE 225 



ant types. These facts are due to the different hereditary back- 

 grounds on which the modifying conditions act. 



Finally, if one admits the above generalizations to be true, 

 studies and descriptions of individual monsters and deformities 

 lose much of their interest and so-called value. It is evident 

 from the present standpoint that a single deformed specimen, 

 whether human or lower vertebrate, must be considered as result- 

 ing from an arrest or slowing of its developmental rate during a 

 particular period. Observing the nature of the deformity or the 

 parts involved enables one to estimate the developmental period 

 during which the arrest was effective. Such individual monsters 

 in no way supply evidence to determine what the initiating cause 

 of the deformities may have been, since we know that the same 

 type of deformity may be experimentally caused by many differ- 

 ent treatments. We can estimate simply that the exciting cause 

 acted to lower the rate of development during a definite interval. 



The great number of descriptions in the literature of isolated 

 monsters have added very little to our understanding of the 

 causes of abnormal development. The writer believes after a 

 prolonged study of this subject that the only benefit to be derived 

 from examinations of such isolated specimens is possibly to ob- 

 tain aid in studying the normal sequence of development. This 

 of course is valuable and only to this extent are such descriptions 

 worthy of record. Detailed descriptions of monsters occupy the 

 same level of scientific value as do records of ordinary structural 

 anomalies observed in the dissecting room. 



Having stated the above conclusions derived from an exten- 

 sive study of abnormalities in single individuals as well as the 

 double specimens under present considerations, it is not deemed 

 necessary to enter into a general discussion of the various views 

 regarding abnormal development with which morphological lit- 

 erature abounds. Many of these positions have been discussed 

 in my previous papers. I shall here only attempt to consider 

 briefly the last contribution by Mall ('17) who devoted so much 

 study and masterly consideration to this subject. The inves- 

 tigations by Mall are far the most valuable that have been 

 made on human material. 



