276 The Phenomena of Morphogenesis 



duction of different forms of leaves on the same plant or the growth 

 of plantlets on the leaf margins or many structures appearing during the 

 process of regeneration are normal or not. What seems at first to be ab- 

 normal may prove to be simply an intensified or exaggerated manifesta- 

 tion of developmental potency. The degree of divergence from the 

 average is the basis on which we term a structure or an individual ab- 

 normal, and biologists often disagree as to how divergent an organism 

 must be to warrant this designation. "Abnormalities" in animals and 

 plants are recognized largely as a matter of convenience in order not to 

 complicate still further an already difficult taxonomic and morphological 

 situation. 



Thus there are two concepts of what a norm is and therefore of what 

 is abnormal. One is a developmental concept: the norm or standard, 

 based on a specific protoplasmic pattern, to which the organism tends 

 persistently to conform. The existence of this norm is the basic fact in 

 biological organization. Its expression may vary greatly as the environ- 

 ment changes but it always remains as the core of the morphogenetic 

 process. In this sense, nothing is abnormal. The other concept of the 

 norm is a purely statistical or taxonomic one. In most species the de- 

 velopmental norms of its individuals do not vary widely in their expres- 

 sion since these individuals are genetically very similar and have been 

 exposed to a relatively narrow range of environmental influences. This 

 rather constant developmental expression may for convenience be re- 

 garded as a norm, and everything that differs from it substantially may 

 be called abnormal. It is in this statistical sense that the term "abnormal 

 growth" is generally employed. 



The student of morphogenesis, however, does not put aside these in- 

 stances of abnormal growth and development as unimportant for his 

 purpose. Such may well prove to be more enlightening than most "nor- 

 mal" individuals. They are exceptions, extreme cases, and from exceptions 

 like these often come clues to the solution of particularly difficult prob- 

 lems. Furthermore, in many cases of abnormal growth certain levels of 

 the very quality that we associate with life— organization— have disap- 

 peared. Tissue cultures and many tumors and galls are formless, largely 

 unorganized masses of cells which no longer produce the beautifully 

 coordinated structures called organisms. Individual cells here must still 

 retain a basic, vital organization in their living stuff, for otherwise they 

 would die, but the higher levels of organization have now broken 

 down. In other types of abnormal growth quite the opposite change 

 has occurred and entirely new structures, specifically formed and well 

 organized though on a different plan, are produced, as in many insect 

 galls. 



The subject of abnormal growth is therefore a promising one for stu- 



