CHAPTER XI 



ANIMAL ANOMALIES 



It is rather generally understood among students of biology that 

 no two animals, even of the same species, are exactly counterparts of 

 each other. There is a certain normal range of variation in size and 

 structure as well as in functional efficiency. Any variations beyond 

 these rather fixed limits are classified as malformations or anomalies. 

 A study of such abnormalities is known as teratology. Abnormalities 

 may occur at almost any stage in the life of the individual, but a 

 large majority result from abnormalities in the process of develop- 

 ment. Many are due to failure of development and some to over- 

 development. In turn, much of this is due to malregulation resulting 

 from failure of balance in the functioning of the endocrine system. 

 The occurrence of striking malformations in newborn human babies is 

 in approximately the ratio of one to 165. Fortunately, many of the 

 most grotesque anomalies do not reach full development and the mon- 

 sters are born dead. 



Causes of Anomalies. — There are both internal and external agen- 

 cies which bring about malformations in the individual. Both em- 

 bryology and pathology contribute to the explanation of the causes 

 of these abnormal conditions. The development may be perfectly 

 normal and a subsequent disease may be the cause of striking ab- 

 normality. On the other hand, certain diseases of the parent will 

 influence the normal development of the fetus. Even twinning by 

 itself is an abnormal process in most animals. It, however, is not 

 usually thought of under the title of anomaly, except when they are 

 physically connected or the individuals are otherwise malformed. 

 The causes of anomalies mav be summarized as follows : 



1. Internal Causes. 



a. The germ plasm carries hereditary causes for some, and these 

 characteristics are transmitted as are normal traits. Color blind- 

 ness and hemophilia (bleeder condition) are examples. 



b. Diseases which cause abnormal growths and conditions, as 

 elephantiasis. 



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