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, 
