140 DOMESTICATED ANIMALS AND PLANTS 



Abnormal growths. Not only do unit characters occasionally 

 get mixed up and jumbled together in quite remarkable fashion, 

 but in rare cases growths occur which, if not formless, at least 

 are in no sense of the term normal body growths. Often these 

 are distorted imitations of the real part, as on the deer's head 

 shown in Fig. 24 ; but often, if not commonly, they are over- 

 growths of some part of the body, induced possibly by irritation 

 or perhaps by poisons, as in galls, in the characteristic tubercle 

 of the disease known as tuberculosis, and in its namesake, the 

 tubercle of the legumes. 



Of this general character, too, is the tumor, that perverse 

 overgrowth due to disorders not understood, but which, from the 

 fact that they " have no typical termination," are not only 

 extremely troublesome but often dangerous to life. 



With this glimpse at the abnormal we are prepared to resume 

 the normal and to discuss briefly how unit characters behave in 

 transmission. 



Summary. Development may go wrong in several ways. First, some 

 part may not develop at all, or, on the other hand, it may far exceed its 

 normal size or function. A part may even be doubled, two parts may fuse 

 into one, or normal characters may get misplaced. The whole organism may 

 exceed the normal size or it may stop short of the usual, and in rare cases 

 abnormal growths may occur in almost any part of the body. 



Exercise. Make collections of plants or parts of plants, including fruits 

 and flowers, in which development has been in some way unusual. Such a 

 collection is not representative of life processes, but it does show what may 

 possibly happen when development goes wrong, and it fixes the conception 

 of unit characters. 



Learn by observation and inquiry all that you can about unusual animals 

 in the neighborhood, either in regard to color markings or abnormal parts, 

 getting photographs and accurate descriptions wherever possible. Let a col- 

 lection of such specimens and photographs accumulate in the school for 

 future studies in abnormal behavior during differentiation. 



